Trace latency: fix writer properties, cache shape, vacuum safety, and eval scenario partitioning

Parent: #281

# Goal

Fix the correctness and file-layout issues that can invalidate later performance work.

This includes writer-property consistency, result-cache key shape, vacuum safety, table maintenance coverage, and the first layout for `eval_scenarios`.

# Scope

- Create a shared writer-properties helper for DataFusion/Delta table writes.
- Make writer profiles explicit per table type: trace spans, trace summaries, GenAI trace tables, Bifrost datasets, dispatch records, control tables where relevant, and eval scenarios.
- Make sure tables that need bloom filters, compression, row-group sizing, and column encodings actually get those writer properties when written and optimized.
- Audit `trace_dispatch`; either add writer properties and maintenance hooks or document why it is intentionally exempt.
- Fix unsafe vacuum behavior so one pod cannot remove files still needed by another pod’s active snapshot.
- Fix result-cache key shape and weights before TTL changes. Cache keys need the full query shape, not just a partial set of inputs.
- Ship `eval_scenarios` as a partitioned Delta table using a created-date partition derived from `created_at`. This table has not shipped, so no migration is needed.

# High-level design

The shared writer helper should remove copy/paste drift between engines. It should not force every table into the same physical layout. Different tables have different query patterns, so the helper should expose table profiles rather than a single global set of Parquet knobs.

`eval_scenarios` should be corrected before release. Use a dedicated date partition column derived from `created_at`, with a stable type suitable for Hive-style partition pruning.

# Acceptance criteria

- Writer properties are built through one shared path or explicitly documented as intentionally different.
- Trace summaries, trace spans, GenAI trace tables, Bifrost tables, dispatch records, and eval scenarios have an explicit maintenance/layout decision.
- `eval_scenarios` writes include a created-date partition derived from `created_at`.
- Result-cache keys include the full query shape required for correctness.
- Vacuum behavior is safe under multi-pod readers.
- Tests cover writer-property construction and the `eval_scenarios` partition column.