feat(search): add ranked post id pool API

[rbcorrales]

Summary

Adds Search::post_ids() for consumers that need a ranked pool of unique post IDs and want to hydrate inside their own WP_Query context.

1. Adds Search::MAX_POOL, capped at 200.
2. Reuses dense, sparse, and hybrid ranking.
3. Collapses raw chunk rows by doc_id before hydration.
4. Keeps Search::run() unchanged, including the public 20 result cap and response shape.

Manual test plan

1. On a site with indexed posts, invoke Search::post_ids() from a small test plugin, a theme snippet, or wp eval with mode values dense, sparse, and hybrid.
2. Confirm each call returns a plain ordered list of post IDs rather than enriched result rows.
3. Confirm a post with multiple matching chunks appears once, at the position of its best ranked chunk.
4. Confirm requesting a pool larger than 200 returns no more than 200 IDs.
5. Confirm a caller can run a perm => readable hydration pass and preserve the returned ranking with orderby => post__in.
6. Confirm a normal Smart Search or Ability request that uses Search::run() still returns the existing result payload.

[Copilot]

Pull request overview

This PR adds a new consumer-facing API, WPVDB_Search\Search::post_ids(), to return an ordered, de-duplicated pool of ranked post IDs (for callers that want to hydrate results in their own WP_Query context) while keeping Search::run() behavior and payload unchanged.

Changes:

• Introduces Search::post_ids() and Search::MAX_POOL (200), reusing existing dense/sparse/hybrid ranking and collapsing chunk rows to unique doc_ids.
• Adds a private helper collapse_raw_post_ids() to de-duplicate merged chunk rows while preserving rank order.
• Updates unit tests, test bootstrap stubs, and README documentation to cover and describe the new API.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
includes/class-search.php	Adds MAX_POOL, implements post_ids(), and adds collapse_raw_post_ids() to return ranked unique post IDs.
tests/Unit/SearchTest.php	Adds unit tests covering raw collapse behavior, post ID pools across modes, and pool clamping.
tests/bootstrap.php	Extends WP/WPVDB stubs to support fulltext checks, embeddings, and SQL/json helpers used by the new path.
README.md	Documents Search::post_ids() usage, arguments, max pool cap, and hydration/visibility responsibilities for callers.

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