feat(rag): add deterministic local document index

[adihex]

Adds a corpus-free in-memory indexing and lexical retrieval adapter for the provider-independent RAG runtime.

• caller-owned stable document IDs and metadata
• collision-free deterministic chunk IDs
• idempotent full rebuild semantics
• deterministic token-overlap ranking
• no network, model, database, or corpus dependency

Stacked on #21.

Validation:

• 17 Python unit tests passed
• uv build --wheel succeeded
• mise exec -- pnpm build succeeded in a clean detached worktree

[adihex]

Landed on main as part of the consolidated, rebased stack ending at e9bd4bd. Verified this PR has no patches absent from origin/main, so closing the now-redundant stacked PR.