refactor(#3309): extend planning-snapshot.cts with 8 more parsed fields

Phase 11 of epic #3180 (ADR-3180 §8.1 rule 2). PlanningSnapshot grows from
7 fields to 15: projectSections, statePhaseTokens, stateStatus,
roadmapDeclaredPhases, roadmapPhaseCheckboxes, researchValidationStatus,
milestoneArchiveStatus, planningRootFiles.

Every field is a reused owner (buildRoadmapPhaseVariants/
buildNotStartedPhaseVariants from src/validate.cts, stateFieldValue) or a
small relocation of already-working verify.cts logic (PHASE_NUMBER_TOKEN_SOURCE
scanning, the checkMilestonePrefixMismatches sectionRx walk, W009/W018's
file-existence checks) — never a new algorithm, and never raw document text:
§8.1 rule 2 forbids exposing raw text, not exposing a parsed list or boolean
derived from it once by the snapshot builder.

roadmapPhaseCheckboxes deliberately reads the same ROADMAP checkbox
isPhaseComplete (§7.4, disk-strict) refuses to consult — that owner decides
completion and must not read it; this field only exposes what the checkbox
says, for a diagnostic (W011) whose whole purpose is flagging disagreement.
Not a re-derivation of §7.4, recorded explicitly to prevent that reading.

Adds PROJECT_UNREADABLE to UNUSABLE_REASON (ninth #1879 site), closing a
gap the implementing agent correctly flagged rather than silently leaving
absent-vs-corrupt collapsed for PROJECT.md, matching the STATE_UNREADABLE/
CONFIG_UNREADABLE precedent from this same effort's prior commits.

currentPhaseLabel/statePhaseTokens/stateStatus share one STATE.md read
(buildStateFields) rather than three independent reads.

Additive only — all prior fields and worstScope/buildPhaseSnapshot
unchanged.

tests/unusable-input.test.cjs

@@ -72,7 +72,7 @@ describe('UNUSABLE_REASON', () => {
     // (enum + call site + this assertion) instead of a silent widening.
     assert.deepStrictEqual(
       Object.keys(UNUSABLE_REASON).sort(),
-      ['CONFIG_UNREADABLE', 'FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'ROADMAP_UNREADABLE', 'STATE_UNREADABLE'],
+      ['CONFIG_UNREADABLE', 'FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'PROJECT_UNREADABLE', 'ROADMAP_UNREADABLE', 'STATE_UNREADABLE'],
     );
     assert.strictEqual(UNUSABLE_REASON.FRONTMATTER_UNTERMINATED, 'frontmatter_unterminated');
   });
@@ -181,6 +181,53 @@ describe('CONFIG_UNREADABLE', () => {
   });
 });
 
+// ─── PROJECT_UNREADABLE: a PROJECT.md that exists but could not be read ──────
+
+describe('PROJECT_UNREADABLE', () => {
+  test('a genuinely unreadable PROJECT.md produces exactly one diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const emitted = emissionsDuring(() => {
+      const wrote = warnUnusableInput({
+        reason: UNUSABLE_REASON.PROJECT_UNREADABLE,
+        source: '/u/project-unreadable.md',
+      });
+      assert.strictEqual(wrote, true);
+    });
+    assert.strictEqual(emitted, 1);
+  });
+
+  test('the same PROJECT.md path reported twice yields one diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const source = '/u/project-unreadable-dedup/PROJECT.md';
+    const emitted = emissionsDuring(() => {
+      const first = warnUnusableInput({ reason: UNUSABLE_REASON.PROJECT_UNREADABLE, source });
+      const repeat = warnUnusableInput({ reason: UNUSABLE_REASON.PROJECT_UNREADABLE, source });
+      assert.strictEqual(first, true);
+      assert.strictEqual(repeat, false, 'same (path, cause) must dedup');
+    });
+    assert.strictEqual(emitted, 1);
+  });
+
+  test('two different PROJECT.md paths are never suppressed as one', () => {
+    _resetUnusableInputWarningsForTests();
+    const emitted = emissionsDuring(() => {
+      warnUnusableInput({
+        reason: UNUSABLE_REASON.PROJECT_UNREADABLE,
+        source: '/u/project-unreadable-a/PROJECT.md',
+      });
+      warnUnusableInput({
+        reason: UNUSABLE_REASON.PROJECT_UNREADABLE,
+        source: '/u/project-unreadable-b/PROJECT.md',
+      });
+    });
+    assert.strictEqual(emitted, 2, 'keying too coarsely would hide a real second fault');
+  });
+
+  test('the reason value is the frozen string "project_unreadable"', () => {
+    assert.strictEqual(UNUSABLE_REASON.PROJECT_UNREADABLE, 'project_unreadable');
+  });
+});
+
 // ─── The discriminator: truncated vs. everything that merely looks like it ───
 
 describe('extractFrontmatter — flags a genuinely truncated frontmatter', () => {