refactor: adopt shared sql-core plan layer from @worlds/sqlite

[EthanThatOneKid]

Summary

Adopts the shared, driver-free SQL plan layer published in @worlds/sqlite@0.6.0 (./sql-core subpath), deleting this repo's duplicated twins: −261 lines.

What is now shared

• sanitizeFtsQuery + FTS stopwords (was a byte-twin copy)
• buildChunkFtsValue (was byte-identical; twin test removed — coverage lives upstream)
• buildSearchResultId (docstring-only drift; upstream version adopted)
• buildIncludeExcludeFilterClauses/generatePlaceholders/ColumnMapping filter-clause helpers
• chunks_fts DDL, sync triggers, and idx_chunks_quad_id emitters

What stays LibSQL-local (by design)

• Native vector dialect: vector F32_BLOB(n) column, idx_chunks_vector, vector32(?)/vector_top_k('idx_chunks_vector', ...) statements
• In-SQL RRF fusion (single-statement keyword+vector CTE family — all three search branches kept verbatim, emitted SQL unchanged)
• Column-per-position quads table + 7 covering indexes
• Legacy FTS migration/backfill statements

Public API unchanged: sanitizeFtsQuery and buildChunkFtsValue remain exported from ./search-index.

Verification

• deno task ci — fmt, lint, check, tests green including the parity suite (libsql agrees with the in-memory reference on the corpus)
• deno task publish:dry — clean
• Version 0.5.0 → 0.6.0 (new runtime dependency on jsr:@worlds/sqlite@^0.6.0)

[EthanThatOneKid]

Status: parked, blocked by the org-wide CI teardown crash (#40). Local \deno task ci\ is fully green (Windows, deno 2.9.5), including the parity suite. Remote CI fails with exit 139 after every test passes — same signature as \main\ since the de-pin (#37/#39), independent of this diff. Will rebase/merge once the Deno resolution strategy lands. Worktree kept at \worktrees/worlds-libsql/adopt-sql-core\ (branch up to date with origin).