test(gates): the snapshot holds a config per scope, not a path per file

gates.snapshot.json listed ~800 file paths under their config, so every
new or deleted frontend file regenerated it (31% of commits, E34 S2) and
the regen diff buried the gate changes it exists to show. Now a scope is
named by path (scopeOf: src, src tests, ui, lib tests, …), the snapshot
holds one effective config per scope (2345 → 1456 lines), and a new test
fails when a file's config is not its scope's.

Not narrowed: a new file in a known place changes nothing (it gets its
scope's config, checked), a glob that moves one file into another config
fails (seen: an override on Feed.tsx → "src: src/components/Feed.tsx"), a
scope whose rules change is a snapshot diff, and a file no config lints
is still a register miss.

CLAUDE.md

@@ -20,7 +20,7 @@ green **before** any merge, release, or handing work to review.
 
 - Full gate: `bash scripts/check.sh` (`lint` for the static lanes only); the private ops dispatcher's `siftlode check` runs the same script. Single lanes: [`docs/development.md`](docs/development.md) § The gate.
 - **There is no CI** (owner, 2026-10-04): this gate, run here, is the only one — green before every hand-over, and `release` refuses to merge without it.
-- **A gate with any real hit is never switched off or narrowed** — not a lint rule, a compiler flag, a lane or a release step. Fix the sites, or suppress each one inline naming its invariant (`-- why`, lint-enforced). What stays narrowed is in [`docs/gates.md`](docs/gates.md), and a contract test fails when a narrowing is missing from it. The effective ESLint config of every file and the gate/lane lines are pinned in `frontend/src/lib/gates.snapshot.json`: a new code file or any gate change fails until you regenerate it (`GATE_SNAPSHOT=update`, see gates.md) — then read that diff as a gate change. A comment saying "X is enforced by Y" is checked by running Y (E32: one such comment hid 95 functions the React Compiler skipped).
+- **A gate with any real hit is never switched off or narrowed** — not a lint rule, a compiler flag, a lane or a release step. Fix the sites, or suppress each one inline naming its invariant (`-- why`, lint-enforced). What stays narrowed is in [`docs/gates.md`](docs/gates.md), and a contract test fails when a narrowing is missing from it. The effective ESLint config of every scope and the gate/lane lines are pinned in `frontend/src/lib/gates.snapshot.json`: any gate change fails until you regenerate it (`GATE_SNAPSHOT=update`, see gates.md) — then read that diff as a gate change. A comment saying "X is enforced by Y" is checked by running Y (E32: one such comment hid 95 functions the React Compiler skipped).
 - **The backend type lane runs in the test image, not on the host** — `requirements.lock` cannot be installed on Windows (uvloop), and a type checker without the real packages reports nonsense. It fails on NEW errors only; the pre-existing ones sit in `backend/.basedpyright/baseline.json` and burn down epic by epic. Fixed some? Regenerate with `--writebaseline`.
 - New DB-backed test? It must actually run, not silently skip — assume `SIFTLODE_TEST_REQUIRE_DB=1`.