perf(C0-PR5): G7/G8/G9/G10 recall hot-path fixes

[m3talux]

Summary

• G7: Hoist all db.prepare() calls into openIndex() so zero statements are prepared per method invocation; eliminates N+1 prepare pattern across hydrate, bm25Global, vectorKNNGlobal, globalFallbackNotes, and all other hot-path methods
• G8: serializeInt8ForSql returns a Buffer over raw int8 bytes instead of a JSON array string; vec_int8(?) accepts a buffer directly, removing per-call JSON serialisation and re-parse on every upsert and KNN query
• G9: Add CREATE INDEX IF NOT EXISTS notes_project and notes_created at openIndex() time (idempotent); globalFallbackNotes now uses an index scan
• G10: Remove daily/ and meta/preferences.md path heuristics from global filter methods; the sole filter is note.project = 'global'. indexInto in indexer.ts stamps project='global' for cross-cutting folders (daily/, decisions/, lessons/, knowledge/, meta/) when frontmatter lacks a project value, preventing global-filter starvation without C1

Test plan

• G7: spy on db.prepare after openIndex(), call each global method 3x, assert zero further prepares
• G8: unit-assert serializeInt8ForSql returns a Buffer with correct byteLength; round-trip upsert/vectorKNN correctness
• G9: assert notes_project and notes_created indexes exist; EXPLAIN QUERY PLAN does not show SCAN TABLE notes for the global fallback query
• G10: a project=global note at a non-daily path is returned; a daily/ note with project=null is excluded
• Indexer: indexNote stamps project=global for each cross-cutting folder; explicit project in frontmatter is preserved
• Full suite green (799/799), tsc --noEmit clean

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