fix: keep FM window scaled during degrade() and add /api/fidelity endpoint

Window was restored before degrade() was called, so FM always saw
NORMAL pressure internally. Now keeps scaled window through the
degrade call. Adds /api/fidelity debug endpoint showing FM state,
object counts, pressure ratios, and fidelity distribution per session.

src/mnemosyne/gateway.py

@@ -1419,30 +1419,27 @@ def _apply_fidelity(payload: dict, session: "Session") -> None:
     # (Same fix as _update_fidelity_pressure — FM's total_tokens() is only
     # a fraction of the real context, so internal pressure is always NORMAL.)
     obj_tokens = fm.total_tokens()
+    saved_ws = fm.window_size
     if obj_tokens > 0 and fm.window_size > 0:
         real_pressure_ratio = session.token_state.get("last_effective", 0) / fm.window_size
         if real_pressure_ratio > 0:
-            saved_ws = fm.window_size
             fm.window_size = max(1, int(obj_tokens / real_pressure_ratio))
-            pressure = fm.current_pressure()
-            fm.window_size = saved_ws
-        else:
-            pressure = fm.current_pressure()
-    else:
-        pressure = fm.current_pressure()
+    pressure = fm.current_pressure()
     if pressure > PressureZone.NORMAL:
+        # Keep scaled window during degrade() so it sees correct pressure
         transitions = fm.degrade(turn)
         for obj_id, old_level, new_level in transitions:
             transitions_logged.append(f"{obj_id[:8]}:{old_level.name}→{new_level.name}")
+    fm.window_size = saved_ws
 
-        if transitions_logged:
-            print(
-                f"  {_DIM}[{session.id}] fidelity: pressure={pressure.name}, "
-                f"degraded {len(transitions_logged)} objects: "
-                f"{', '.join(transitions_logged[:5])}"
-                f"{'...' if len(transitions_logged) > 5 else ''}{_RESET}",
-                file=sys.stderr,
-            )
+    if transitions_logged:
+        print(
+            f"  {_DIM}[{session.id}] fidelity: pressure={pressure.name}, "
+            f"degraded {len(transitions_logged)} objects: "
+            f"{', '.join(transitions_logged[:5])}"
+            f"{'...' if len(transitions_logged) > 5 else ''}{_RESET}",
+            file=sys.stderr,
+        )
 
 
 # ── Gateway factory ──────────────────────────────────────────────────────
@@ -1665,6 +1662,39 @@ def create_app(
             "session_tokens": total_tokens,
         }
 
+    @app.get("/api/fidelity")
+    async def api_fidelity(session_id: str | None = None) -> dict[str, Any]:
+        """Debug endpoint: expose FidelityManager state per session."""
+        all_sessions = sessions.all()
+        out: dict[str, Any] = {}
+        for sid, sess in all_sessions.items():
+            if session_id and sid != session_id:
+                continue
+            fm = sess.fidelity_manager
+            obj_count = len(fm._objects)
+            obj_tokens = fm.total_tokens()
+            dist: dict[int, int] = {}
+            for obj in fm._objects.values():
+                dist[obj.current_fidelity] = dist.get(obj.current_fidelity, 0) + 1
+            last_eff = sess.token_state.get("last_effective", 0)
+            real_ratio = last_eff / fm.window_size if fm.window_size > 0 else 0
+            out[sid] = {
+                "object_count": obj_count,
+                "object_tokens": obj_tokens,
+                "window_size": fm.window_size,
+                "real_pressure_ratio": round(real_ratio, 3),
+                "internal_pressure": fm.current_pressure().name if obj_count > 0 else "NO_OBJECTS",
+                "last_effective_tokens": last_eff,
+                "fidelity_distribution": dist,
+                "thresholds": {
+                    "caution": fm.threshold_caution,
+                    "warning": fm.threshold_warning,
+                    "critical": fm.threshold_critical,
+                    "emergency": fm.threshold_emergency,
+                },
+            }
+        return out
+
     @app.get("/api/blocks")
     async def api_blocks(session_id: str | None = None) -> dict[str, Any]:
         """Debug endpoint: expose BlockStore state per session."""