docs: add .claude-context overview with session log

.claude-context/00-overview.md

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+# Newbury Exhibit — Overview
+
+Fan-made shared multi-viewer AR experience for a physical LEGO® Hidden Side™ town
+display (tribute project — not affiliated with the LEGO Group). Multiple phones
+point at printed "Newbury Crests" (ArUco markers) and see the same server-driven
+ghosts anchored to real-world positions, each from their own viewpoint.
+
+## Shape of the thing
+
+- **Delivery:** web, no app install. Visitors open a URL and press Start.
+- **Tracking:** printed fiducials + a joint multi-marker solve. No SLAM, no
+  cloud AR service — self-hosted end to end.
+- **Authority:** the server owns which ghosts exist and where; clients own only
+  their own camera pose. That's what keeps multiple phones consistent.
+- **Motion:** deterministic functions of (spawn record, wall-clock time), so
+  phones agree without streaming positions.
+
+## Current state (v3)
+
+v3 is a **tracking-core transplant** into v2's body: the playlist engine,
+residents/tracks, character editor, admin suite, error overlay, password-gated
+final mode and deployment all carry over unchanged. What changed is how the
+camera pose is recovered — see `01-architecture.md`.
+
+Deployed in place of v2: same host port, container name and `exhibit-live`
+volume, so live state carries over and rollback is one command.
+
+- Repo: `newbury-exhibit-v3` (v2 retained as the rollback path)
+- Deploy: `cd ~/newbury-exhibit-v3 && bash deploy.sh`
+- Rollback: `cd ~/newbury-exhibit && docker compose up -d --build`
+- Page index, API list and HUD guide: `docs/PAGES.md`
+
+## Conventions
+
+- World units cm; origin at the front-left table corner; +X right, +Z back,
+  +Y up. Marker print sizes stay in mm (`sizeMM`), converted once at the solver
+  boundary.
+- `data/*.json` are committed seeds; `data/live/` (a Docker volume) is runtime
+  state.
+- Dev mode shows a tracking HUD and an on-screen error overlay; final mode
+  password-gates admin.
+
+## Session Log
+
+- 2026-09-23 — AR Tracking Overhaul: Worker Offload, Camera Model, and Coplanar Pose Ambiguity — moved OpenCV off the main thread, fixed the focal/FOV model for portrait streams, deduped marker ids, gated the mirrored coplanar pose, and built a two-page A3 test rig with a foldable marker tower.