path query: SQLite step index (path-cli 0.18.0)

[benbaarber]

Stacked on #138. Implements Approach A from docs/design/query-acceleration.md (committed here, with measured results).

What

A disposable SQLite accelerator at $CONFIG_DIR/index.db (query/index.rs):

• Every wrapped step as a row — compact JSON, exactly what wrap_graph emits — plus VIRTUAL generated columns (dead_end, actor, source) with per-doc composite indexes.
• Stat-gated freshness: each doc records its file's (mtime_ns, size); rows serve only on a fresh match, otherwise the file reparses and reindexes in place (stamp taken before the read, so racing writes re-derive next query). Cache files stay canonical: delete the index and it rebuilds lazily; a schema-version bump rebuilds automatically; TOOLPATH_QUERY_NO_INDEX=1 bypasses.
• Write-through: cache::write_cached indexes what it writes (sync/import keep the index warm); p cache rm purges; full scans prune orphans.
• Predicate recognition (plan.rs::RowPredicate, whole-predicate-or-nothing): .dead_end (and | not), .step.actor == "…", .step.actor | startswith("…"), .path.meta.source == "…", and and-conjunctions — from a leading map(select(P)) / .[] | select(P) / [.[] | select(P)] stage.
• Two accelerated shapes: a filter that is exactly length or map(select(P)) | length collapses to SELECT count(*) (no step parsed at all); any recognized leading select prefilters rows in SQL before parsing, with the unchanged original filter still running on survivors.
• Content-scoped queries (--kind/--project/--parent-dir) bypass the index (those filters live in the wrapping pass). --input/stdin are never indexed. emscripten build compiles without any of this.

Measured (real 97-doc / 114 MB / 46k-step cache, medians of 5)

Query	baseline	rayon #138	+ index	output
length	898 ms	292 ms	30 ms	IDENTICAL
map(select(.dead_end)) | length	957 ms	309 ms	30 ms	IDENTICAL
map(select(.step.actor | startswith("agent:"))) | length	976 ms	310 ms	169 ms	IDENTICAL
.[] | select(.dead_end) | .step.id	937 ms	304 ms	311 ms	IDENTICAL
map(.step.actor) | unique (slurp)	1021 ms	856 ms	881 ms	IDENTICAL
group_by(.path.id) | map(length) | max (slurp)	1032 ms	879 ms	902 ms	IDENTICAL
--source claude 'length'	548 ms	200 ms	28 ms	IDENTICAL

Absorbed counts are flat (~30 ms) regardless of cache size/shape (same
on the 56 MB synthetic cache). Honest costs and limits: one-time index
build ~1.7 s on first query; index size ~250 MB beside the 114 MB
cache; a predicated stream lands at parity with #138's parallel scan
(row-fetch + per-row parse ≈ parallel whole-file parse), and
unpredicated slurps and streams stay within a few percent of #138.
rm ~/.toolpath/index.db reverts at any time.

Re-measured 2026-07-23: the cache previously held a single 110 MB
document (48% of the bytes); with it, the predicated dead-end stream
was slower than the parallel scan (724 ms vs 615 ms — its surviving
rows carried the fat diffs) and absorbed counts showed ~46× against
the fatter baseline. That document has since been deleted.

Output unchanged

The integration suite (tests/query.rs) asserts index-served output is byte-identical to TOOLPATH_QUERY_NO_INDEX=1 across every plan shape — absorbed counts, predicated decompose/stream, predicated and unpredicated slurp, --source scoping, --raw — on first (indexing) and second (serving) runs, plus staleness self-healing (file changed/removed/rewritten behind the index) and p cache rm purging. Unit tests cover the store (round-trip, stamps, replace, purge, retain, empty docs, version rebuild) and the recognizer (recognized shapes, conservative refusals, the in-code matches mirror).

Verification

• scripts/quality_gates.sh: 7/7 pass
• cargo check --target wasm32-unknown-emscripten -p path-cli: clean, no new warnings
• path-cli: 371 unit + 41 query-integration tests pass

Also included: docs/design/query-acceleration.md (the design this implements, updated with measured results) and docs/design/graph-query-frontend.md (the facto/GraphJQ exploration it cross-references).

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