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.

src/unusable-input.cts

@@ -57,6 +57,13 @@ const UNUSABLE_REASON = Object.freeze({
    * silent degradation is visible. (#3099, sixth #1879 site)
    */
   LAST_ACTIVITY_UNPARSEABLE: 'last_activity_unparseable',
+  /**
+   * A STATE.md exists but could not be read (EACCES/EIO/…). Distinct from a project that has
+   * not run `state.init` yet: absence returns the same non-answer, silently — only an
+   * exists-but-unreadable STATE.md is corruption. (#3308, seventh #1879 site —
+   * planning-snapshot's current-phase field)
+   */
+  STATE_UNREADABLE: 'state_unreadable',
 } as const);
 
 type UnusableReason = (typeof UNUSABLE_REASON)[keyof typeof UNUSABLE_REASON];
@@ -69,6 +76,8 @@ const REASON_PROSE: Readonly<Record<UnusableReason, string>> = Object.freeze({
     'ROADMAP.md exists but could not be read; phase and milestone lookups fell back to defaults',
   [UNUSABLE_REASON.LAST_ACTIVITY_UNPARSEABLE]:
     'last_activity in STATE.md is present but unparseable as a date; stale_activity fell back to false (idle-stranded suppressed)',
+  [UNUSABLE_REASON.STATE_UNREADABLE]:
+    'STATE.md exists but could not be read; the current-phase label fell back to unavailable',
 });
 
 // ─── Dedup state ──────────────────────────────────────────────────────────────