Close the plan: screenshots and a measured performance pass

- docs/screenshots: nineteen pages captured with headless Chromium from a
  running instance; README gains a screenshot section
- tests/perf: opt-in load test (NOEMA_PERF=1) seeding 60k events, 6k
  memories and 1.5k beliefs across two minds, timing sixteen hot reads
  against a 400 ms ceiling and rejecting sequential-scan plans
- migration 0016: indexes the test found missing (memories by mind and
  recency, beliefs by mind and confidence, events by mind and category,
  belief versions by mind and time)
- docs/performance.md records the numbers and how to rerun

migrations/0016_hot_path_indexes.down.sql

@@ -0,0 +1,4 @@
+DROP INDEX IF EXISTS belief_versions_mind_created_idx;
+DROP INDEX IF EXISTS cognitive_events_mind_category_seq_idx;
+DROP INDEX IF EXISTS beliefs_mind_confidence_idx;
+DROP INDEX IF EXISTS memories_mind_recent_idx;