[Feature] Track end-to-end `TIMESTAMP_NS` data type support

## Description

This issue tracks the work required to make `TIMESTAMP_NS` a complete, first-class Apache Doris data type, covering SQL syntax, FE/BE type systems, storage, query execution, data loading, output protocols, ecosystem integration, compatibility, testing, and documentation.

`TIMESTAMP_NS` has the following semantics:

- Fixed nanosecond precision with exactly nine fractional digits; `TIMESTAMP_NS(p)` is not supported.
- Stored as a signed 64-bit nanosecond count from Unix epoch `1970-01-01 00:00:00`.
- Valid range: `[1677-09-21 00:12:43.145224192, 2262-04-11 23:47:16.854775807]`.
- It is independent of `DATETIMEV2`; `DATETIME(p)` and `DATETIMEV2(p)` continue to support only `p = 0..6`.
- It does not preserve a time-zone identifier or offset.

The basic implementation is provided by https://github.com/apache/doris/pull/66761. The PR was still open when this tracker was drafted.

Documents: https://github.com/apache/doris-website/pull/4091

## Goals

- Support the complete lifecycle of `TIMESTAMP_NS` in Doris-native tables and query execution.
- Keep FE and BE behavior consistent for parsing, rounding, range checking, casts, functions, comparisons, and serialization.
- Preserve the existing behavior of DATE, DATEV2, DATETIME, DATETIMEV2, TIMEV2, and TIMESTAMPTZ.
- Make unsupported features fail explicitly with clear diagnostics.
- Provide complete unit, regression, compatibility.

## Scope boundary

This tracker covers the Doris type contract and all Doris-native execution and storage paths. External connectors and file formats may be implemented in separate PRs, but their support status must be recorded here.

## Progress

### Foundation

- [ ] 🚧 [PR #66761](https://github.com/apache/doris/pull/66761): Olap `TIMESTAMP_NS` support.
- FE/BE primitive types, catalog and Nereids types, and literals.
- BE value, column, data type, serde, storage, key coding, and indexes.
- Basic DDL, INSERT, Stream Load, predicates, operators, and MySQL output.
- Foundational FE unit, BE unit, and regression tests.

The checkbox should be marked complete only after the PR is merged.

## Work items

### 1. Type semantics and metadata

- [x] Finalize literal grammar, canonical formatting, timezone behavior, rounding, overflow, and strict/non-strict semantics.
- [ ] Support FE catalog, Nereids, Thrift/protobuf, edit-log, metadata replay, and FE-BE plan serialization.
- [ ] Return correct type information in DESCRIBE, SHOW CREATE TABLE, information_schema, EXPLAIN, and client metadata.
- [ ] Reject precision arguments and prevent accidental treatment as `DATETIMEV2`.

### 2. DDL and table models

- [ ] Support duplicate, aggregate, unique-key/MoW, and primary-key use cases, as both key and value columns where legal.
- [ ] Support nullable columns, literal defaults, current-timestamp defaults, generated columns, sequence columns, and cluster/sort keys.
- [ ] Support ALTER TABLE, schema change, CTAS, CREATE TABLE LIKE, truncate, clone, and recovery operations.
- [ ] Ensure the FE schema-change matrix matches the conversions implemented by the BE.

### 3. Literals, casts, and type coercion

- [ ] Support normal, invalid, epoch, minimum, maximum, overflow, timezone-offset, and rounding-carry literals consistently in FE and BE.
- [ ] Define and implement casts between `TIMESTAMP_NS` and string, date/time, numeric, JSON, and complex types.
- [ ] Define common-type rules for comparisons, set operations, condition expressions, arrays, maps, and structs.
- [ ] Keep FE folding, BE folding, and runtime execution results identical.

### 4. Date/time functions and expressions

- [ ] Review every function that supports `DATETIMEV2` and explicitly support or reject `TIMESTAMP_NS` with a dedicated signature.
- [ ] Support date/time extraction, formatting, parsing, conversion, truncation, rounding, arithmetic, interval, and difference functions.
- [ ] Support comparison, NULL, BETWEEN, IN/NOT IN, CASE, IF, COALESCE, and related conditional expressions.
- [ ] Preserve nanoseconds unless the function or target type explicitly defines precision loss.

### 5. Query operators

- [ ] Support filtering, projection, aggregation, GROUP BY, DISTINCT, aggregate state, ORDER BY, TOP-N, and window functions.
- [ ] Support all applicable join strategies and null-safe equality.
- [ ] Support hash distribution, shuffle, exchange, spill, and runtime filters.
- [ ] Ensure equality, ordering, hashing, and serialization remain consistent across distributed execution.

### 6. Partitioning, bucketing, and predicate pushdown

- [ ] Support range/list/auto/dynamic partitioning, NULL ownership, boundary values, and partition pruning.
- [ ] Support hash bucketing and bucket pruning with stable hashing for values before and after the epoch.
- [ ] Support short keys, key coding, scan-key splitting, storage predicates, predicate pushdown, and DELETE conditions.
- [ ] Keep existing DATE/DATETIME partition names and pruning behavior unchanged.

### 7. Storage and indexes

- [ ] Support segment/rowset encoding, row store, memtable flush, compaction, partial update, and schema evolution.
- [ ] Support zone map, bloom filter, inverted index, and other legal indexes for writing, reading, and filtering.
- [ ] Verify signed chronological ordering across the epoch in keys, indexes, and min/max statistics.
- [ ] Support clone, migration, repair, backup/restore, replication, and cloud storage paths.

### 8. Import and write paths

- [ ] Support INSERT, INSERT SELECT, CTAS, overwrite, group commit, and prepared/batch writes.
- [ ] Support Stream Load, Broker Load, Routine Load, MySQL Load, and supported file/TVF ingestion paths.
- [ ] Support strict/non-strict mode, error filtering, partial update, transaction rollback, retry, and replay.
- [ ] Cover values with and without timezone offsets, invalid values, NULLs, bounds, and rounding overflow.

### 9. Materialized views and generated columns

- [ ] Support synchronous materialized views, asynchronous MVs, and MTMVs.
- [ ] Support MV partition mapping, refresh, transparent rewrite, and compensation predicates.
- [ ] Support generated columns that consume or return `TIMESTAMP_NS`.
- [ ] Ensure existing DATETIMEV2 partition formatting is not changed by the new type.

### 10. Complex types and aggregate state

- [ ] Support `TIMESTAMP_NS` in ARRAY, MAP, STRUCT, JSON/JSONB, and VARIANT where applicable.
- [ ] Support nested-type storage, load, shuffle, comparison, and output.
- [ ] Support applicable aggregate functions and aggregate-state serialization/merge.
- [ ] Reject unsupported combinations during analysis with clear errors.

### 11. Client protocols, export, and ecosystem

- [ ] Support MySQL text and binary/prepared-statement protocols with consistent metadata and value encoding.
- [ ] Define JDBC/ODBC and common client-driver behavior.
- [ ] Support or explicitly reject CSV, Parquet, ORC, Arrow, OUTFILE, EXPORT, and Arrow Flight SQL paths.
- [ ] Define mappings for external catalogs, Spark/Flink connectors, and supported UDF frameworks.
- [ ] Never silently convert nanoseconds to microseconds in an external interface.

### 12. Statistics and optimizer

- [ ] Support min/max, NDV, histograms, partition statistics, statistics persistence, and SHOW COLUMN STATS.
- [ ] Support filter/join estimation, materialized-view rewrite, plan cache, SQL cache, and prepared-statement reuse.
- [ ] Verify partition, bucket, index, TOP-N, and runtime-filter pruning.

### 13. Compatibility and operations

- [ ] Keep new enum and storage identifiers append-only and preserve existing date/time encodings.
- [ ] Verify that shared parsing, formatting, coercion, function, and partition code does not change existing types unintentionally.
- [ ] Define rolling-upgrade, mixed-version, downgrade, restart/replay, backup/restore, CCR, and cloud behavior.
- [ ] Add a capability/version check so incompatible BEs do not receive `TIMESTAMP_NS` metadata or plans.
- [ ] Make unsupported paths fail clearly rather than crash or corrupt data.

### 14. Testing and documentation

- [ ] Add FE unit tests for type validation, literals, casts, coercion, folding, metadata, pruning, functions, and compatibility.
- [ ] Add BE unit tests for values, columns, serde, storage encodings, indexes, predicates, casts, functions, and operators.
- [ ] Add purpose-based regression suites for table models, partition/bucket pruning, indexes, import, expressions, functions, joins, aggregation, MV, complex types, and output.
- [ ] Cover normal, invalid, boundary, overflow, epoch, nullable, timezone, rounding, restart, failure-recovery, cloud, and non-cloud cases.
- [ ] Publish syntax, semantics, cast/function matrices, limitations, client/format support, and upgrade documentation.

## Correctness and compatibility checklist

- [ ] FE, BE, storage, loading, and output use identical parsing, rounding, and range rules.
- [ ] Equality, ordering, hashing, partitioning, indexing, joins, and runtime filters agree for the same value.
- [ ] Values before 1970 retain correct signed chronological ordering.
- [ ] NULL and range-boundary behavior is correct in partitions, predicates, joins, and indexes.
- [ ] No supported path silently loses nanosecond precision.
- [ ] Existing date/time types retain their previous syntax, range, formatting, casts, functions, partition behavior, and output.
- [ ] Unsupported and mixed-version paths fail safely with clear diagnostics.

## Exit criteria

- `TIMESTAMP_NS` works end to end for all supported internal table models and DDL/DML operations.
- Casts, coercion, functions, expressions, query operators, storage, indexes, loading, and output are implemented or explicitly rejected.
- Partitioning, bucketing, pruning, joins, runtime filters, aggregation, MVs, generated columns, and complex types are verified.
- Upgrade, downgrade, cloud, backup/restore, replication, and failure-recovery behavior is defined and tested.
- FE unit, BE unit, regression, compatibility, and coverage gates pass.
- User-facing documentation and all remaining limitations are published. https://github.com/apache/doris-website/pull/4091

## Related work

- Foundation PR: [PR #66761](https://github.com/apache/doris/pull/66761)