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

README.md

@@ -38,6 +38,16 @@ Noema is also not a chatbot with a memory bolted on. A user message does not go
 
 Full description: [ARCHITECTURE.md](ARCHITECTURE.md). Decisions: [docs/adr](docs/adr). Plan and status: [PLAN.md](PLAN.md).
 
+## Screenshots
+
+Captured from a running instance with the sample mind Dani (full set in [docs/screenshots](docs/screenshots)).
+
+| | |
+|---|---|
+| ![Mind overview](docs/screenshots/mind.png) | ![Thought stream](docs/screenshots/thoughts.png) |
+| ![Beliefs](docs/screenshots/beliefs.png) | ![Relationship graph](docs/screenshots/graph.png) |
+| ![Cognitive replay](docs/screenshots/replay.png) | ![Policies](docs/screenshots/policies.png) |
+
 ## Quick start
 
 Requirements: Go 1.26+, Docker with Compose, `make`.