[Enhancement] Reduce memory and allocation overhead of DUP_KEYS MemTables

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Description

DUP_KEYS MemTables currently create one `shared_ptr` for every inserted row, even though these rows only require their positions for sorting.

This introduces substantial per-row metadata overhead, frequent heap allocations, and unnecessary allocation churn during large data loads. The overhead becomes
especially significant when a MemTable contains a large number of rows.

We can use a more compact representation for DUP_KEYS while preserving the existing sorting semantics:

- Sort rows by key in ascending order.
- For equal keys, sort by row position in descending order.
- Keep the existing behavior for UNIQUE_KEYS and AGG_KEYS unchanged.

### Solution

Replace the per-row `shared_ptr` objects used by DUP_KEYS MemTables with a contiguous `uint32_t` row-position vector.

The row positions are reserved before rows are appended to the mutable block and populated using `std::iota`, avoiding additional allocations after the block has been
modified. The existing `RowInBlock` representation remains unchanged for UNIQUE_KEYS and AGG_KEYS because those models require aggregation state.

This optimization reduces explicit row-index metadata from at least dozens of bytes per row to 4 bytes per row, eliminates per-row object allocations, and improves
MemTable insertion efficiency without changing query or load semantics.

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Locate the DUP_KEYS MemTable insertion and sorting entry points, then compare them with the unchanged UNIQUE_KEYS and AGG_KEYS paths. Verify that row positions are reserved and populated before the mutable block is modified, and run the relevant MemTable tests to confirm ascending key order, descending row-position order for equal keys, and unchanged behavior for the other models.

Written by the indexing model from the issue text.