refactor(#3308): add planning-snapshot.cts parsed projection per ADR-3180 §8.1

Phase 10 of epic #3180. src/planning-snapshot.cts is a new parsed
projection of .planning/, composed exclusively from the already-
consolidated §7 owners (getMilestoneInfo, listMilestonePhaseDirs,
isPhaseComplete, scanPhasePlans, stateFieldValue, planningPaths) plus
the frozen SCOPE enum. No new semantic derivation is introduced beyond
worstScope, a pure combinator folding several independently-scoped
owner answers into one composite signal.

Adds STATE_UNREADABLE to src/unusable-input.cts's UNUSABLE_REASON
(seventh #1879 site) for STATE.md exists-but-unreadable, distinct
from absent.

Adds scripts/lint-planning-snapshot-bypass-drift.cjs, a ratcheted
drift guard (ADR-3180 Decision 4(e)) scoped to DIAGNOSTIC_RULE_FUNCTIONS
(currently cmdValidateHealth in src/verify.cts only) preventing new
raw .planning/ reads from bypassing the snapshot, while acknowledging
cmdValidateHealth's existing 15 raw-read sites as debt owned by
Phase 11 (#3309).

Six-gate .cts ripple: .gitignore, eslint.config.mjs,
docs/INVENTORY.md + manifest regen, CONTEXT.md glossary entry.

Breaking changes: none. This phase adds the subject only; Phase 11
migrates cmdValidateHealth onto it.

docs/INVENTORY-MANIFEST.json

@@ -423,6 +423,7 @@
       "plan-drift-guard.cjs",
       "plan-scan.cjs",
       "planning-scope.cjs",
+      "planning-snapshot.cjs",
       "planning-workspace.cjs",
       "probe-core.cjs",
       "profile-output.cjs",