feat(memory): add retention model test gaps and health diagnostics

Wave 1 - P0 Test Gaps:
- Add hard stale prune removed regression test
- Add dormant overlap tests (entry created during dormancy)
- Add invalid timestamp NaN protection test
- Add reinforcement ordering test with reference type
- Add dedupe same-session/under-1hr guard tests
- Fix NaN handling with Number.isFinite check

Wave 2 - Helper Functions:
- Add timestampMs() for safe timestamp conversion
- Add isSafetyCriticalForDiag() aligned with runtime

Wave 3 - Health Output Format:
- Fix top rendered candidates sorted by strength (not text length)
- Add stored vs rendered counts breakdown
- Add type caps and global cap overflow display
- Track globalCapped array explicitly
- Add dormant status section

Wave 4 - Monitoring Metrics:
- Add high_importance_ratio (alert > 30%)
- Add safety_critical_count (alert > 5)
- Add max_reinforced_count (alert > 10% active)

Wave 5 - Integration Fixture:
- Add 34-entry over-cap test
- Add mixed retention regression fixture
- Test TYPE_MAX caps, safety-critical exemption, reinforcement ordering

Tests: 224 → 237

src/workspace-memory.ts

@@ -67,6 +67,11 @@ export function calculateEffectiveHalfLife(memory: LongTermMemoryEntry): number
   return BASE_HALF_LIFE_DAYS / factor;
 }
 
+function timestampMs(value: unknown, fallback: number): number {
+  const ms = typeof value === "number" ? value : new Date(String(value)).getTime();
+  return Number.isFinite(ms) ? ms : fallback;
+}
+
 export function calculateRetentionStrength(
   memory: LongTermMemoryEntry,
   now: number,
@@ -76,14 +81,16 @@ export function calculateRetentionStrength(
   const effectiveHalfLife = calculateEffectiveHalfLife(memory);
 
   // Use retentionClock if available, fallback to updatedAt.
-  const retentionStart = memory.retentionClock ?? memory.updatedAt;
-  const createdAtMs = new Date(retentionStart).getTime();
+  const retentionStart = Number.isFinite(memory.retentionClock)
+    ? memory.retentionClock
+    : memory.updatedAt ?? memory.createdAt;
+  const createdAtMs = timestampMs(retentionStart, now);
   const effectiveAgeDays = calculateEffectiveAgeDays(createdAtMs, now, lastActivityAt);
 
   // Calculate strength using exponential decay.
   const strength = initialStrength * Math.pow(2, -effectiveAgeDays / effectiveHalfLife);
 
-  return Math.max(0, strength);
+  return Number.isFinite(strength) ? Math.max(0, strength) : 0;
 }
 
 export function calculateDormantDays(store: WorkspaceMemoryStore, now: number): number {