test(#3308): failing-first tests for planning-snapshot parsed projection

ADR-3180 epic #3180 Phase 10 (§8.1): tests for the not-yet-existing
src/planning-snapshot.cts (buildPlanningSnapshot, worstScope), the
not-yet-existing scripts/lint-planning-snapshot-bypass-drift.cjs guard,
and the new STATE_UNREADABLE reason on tests/unusable-input.test.cjs's
already-shipped UNUSABLE_REASON enum. RED by construction: the modules
under test do not exist yet.

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(),
-      ['FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'ROADMAP_UNREADABLE'],
+      ['FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'ROADMAP_UNREADABLE', 'STATE_UNREADABLE'],
     );
     assert.strictEqual(UNUSABLE_REASON.FRONTMATTER_UNTERMINATED, 'frontmatter_unterminated');
   });
@@ -87,6 +87,53 @@ describe('UNUSABLE_REASON', () => {
   });
 });
 
+// ─── STATE_UNREADABLE: a STATE.md that exists but could not be read ─────────
+
+describe('STATE_UNREADABLE', () => {
+  test('a genuinely unreadable STATE.md produces exactly one diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const emitted = emissionsDuring(() => {
+      const wrote = warnUnusableInput({
+        reason: UNUSABLE_REASON.STATE_UNREADABLE,
+        source: '/u/state-unreadable.md',
+      });
+      assert.strictEqual(wrote, true);
+    });
+    assert.strictEqual(emitted, 1);
+  });
+
+  test('the same STATE.md path reported twice yields one diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const source = '/u/state-unreadable-dedup/STATE.md';
+    const emitted = emissionsDuring(() => {
+      const first = warnUnusableInput({ reason: UNUSABLE_REASON.STATE_UNREADABLE, source });
+      const repeat = warnUnusableInput({ reason: UNUSABLE_REASON.STATE_UNREADABLE, source });
+      assert.strictEqual(first, true);
+      assert.strictEqual(repeat, false, 'same (path, cause) must dedup');
+    });
+    assert.strictEqual(emitted, 1);
+  });
+
+  test('two different STATE.md paths are never suppressed as one', () => {
+    _resetUnusableInputWarningsForTests();
+    const emitted = emissionsDuring(() => {
+      warnUnusableInput({
+        reason: UNUSABLE_REASON.STATE_UNREADABLE,
+        source: '/u/state-unreadable-a/STATE.md',
+      });
+      warnUnusableInput({
+        reason: UNUSABLE_REASON.STATE_UNREADABLE,
+        source: '/u/state-unreadable-b/STATE.md',
+      });
+    });
+    assert.strictEqual(emitted, 2, 'keying too coarsely would hide a real second fault');
+  });
+
+  test('the reason value is the frozen string "state_unreadable"', () => {
+    assert.strictEqual(UNUSABLE_REASON.STATE_UNREADABLE, 'state_unreadable');
+  });
+});
+
 // ─── The discriminator: truncated vs. everything that merely looks like it ───
 
 describe('extractFrontmatter — flags a genuinely truncated frontmatter', () => {