fix(memory): ScreenMemory threshold hardening + stale layout purge (P0-3)

- Raised similarity_threshold from 0.90 to 0.95 in get_screen_type() to eliminate
  POST_DETAIL false-positives caused by embedding saturation.
- Implemented purge_stale_screens() with 24h TTL to remove layout drift.
- Added collection size cap (200 points) to store_screen() with automatic wipe
  when saturated to clear overfitting bias.
- Triggered purge_stale_screens() on ScreenIdentity initialization.

TDD: 3 new tests for threshold defaults and purge method integrity.

tests/core/test_screen_memory_hardening.py

@@ -0,0 +1,68 @@
+"""
+P0-3: ScreenMemory Cache Poisoning Guard
+
+TDD RED: These tests enforce that:
+1. The similarity threshold for screen type cache hits is raised to 0.95
+   to prevent false-positive POST_DETAIL matches.
+2. A purge_stale_screens() method exists and removes entries older than
+   a configurable TTL.
+3. The collection has a max size cap to prevent embedding saturation.
+"""
+
+import time
+
+from GramAddict.core.qdrant_memory import ScreenMemoryDB
+
+
+# ═══════════════════════════════════════════════════════
+# P0-3a: Raised Similarity Threshold
+# ═══════════════════════════════════════════════════════
+
+
+class TestScreenMemoryThreshold:
+    """Session log: 34/60 screens classified as POST_DETAIL at 0.92-0.97 similarity."""
+
+    def test_default_threshold_is_095(self):
+        """
+        RED: The default similarity_threshold in get_screen_type is 0.90,
+        which is too low and causes false-positive POST_DETAIL cache hits.
+        GREEN: Raise default to 0.95.
+        """
+        import inspect
+
+        sig = inspect.signature(ScreenMemoryDB.get_screen_type)
+        default = sig.parameters["similarity_threshold"].default
+
+        assert default == 0.95, (
+            f"Expected default similarity_threshold=0.95, got {default}. "
+            f"Threshold 0.90 causes POST_DETAIL over-matching."
+        )
+
+
+# ═══════════════════════════════════════════════════════
+# P0-3b: Staleness Purge
+# ═══════════════════════════════════════════════════════
+
+
+class TestScreenMemoryStalePurge:
+    """Screen layouts change with IG updates; stale cache entries are poison."""
+
+    def test_purge_stale_screens_method_exists(self):
+        """
+        RED: ScreenMemoryDB has no purge_stale_screens() method.
+        GREEN: Add a method that removes entries older than a configurable max_age_hours.
+        """
+        db = ScreenMemoryDB()
+        assert hasattr(db, "purge_stale_screens"), (
+            "ScreenMemoryDB must have a purge_stale_screens() method "
+            "to prevent cache poisoning from stale screen layouts."
+        )
+
+    def test_purge_stale_screens_has_max_age_parameter(self):
+        """The purge method must accept a max_age_hours parameter."""
+        import inspect
+
+        sig = inspect.signature(ScreenMemoryDB.purge_stale_screens)
+        assert "max_age_hours" in sig.parameters, (
+            "purge_stale_screens must accept max_age_hours parameter"
+        )