docs(web): handoff — interactive color-grid web play shipped

HANDOFF.md

@@ -5,7 +5,7 @@
 > the per-CP plan in `docs/superpowers/plans/` + this file.
 
 **Last updated:** 2026-06-02 — CP6 complete (difficulty layouts + category-specific eval +
-human-baseline `compare` harness; offline, headless).
+human-baseline `compare` harness; offline, headless) — Web interactive color-grid play shipped.
 
 ## Done (CP0–CP3)
 
@@ -126,11 +126,22 @@ Executed task-by-task via subagent-driven-development (3-gate: implement → spe
 - **Invariant acceptance:** all four bands print `optimal=up habit=left` with distinct grids `(8,8)/(10,10)/(12,12)/(12,12)` — `optimal != habit` everywhere.
 - **Note:** the `fake:demo` provider plays a fixed non-optimal action (so `motive_reading_accuracy=0`); the smoke validates the deterministic pipeline + metric surfacing, not agent skill.
 
+## Done (Web interactive play — color-grid browser arena; plan: `docs/superpowers/plans/2026-06-02-proteus-web-interactive-play.md`; design: `docs/superpowers/specs/2026-06-02-proteus-web-interactive-play-design.md`)
+
+Executed task-by-task via subagent-driven-development (implement → spec review → code-quality review per task). **Full suite: 148 passed** (the 127 CP6 baseline preserved unchanged + 21 new web/runtime tests).
+
+- **Stdlib-only color-grid web play** — launch with `python -m proteus.web` (`--host`/`--port`, defaults to `127.0.0.1:8000`). A single static `index.html` (vanilla JS, no build, no CDN) renders the engine's integer grids in color via `proteus.arc_grid.rendering.COLOR_MAP`. **Zero new dependencies** — the `.venv` stays SDK-free/offline.
+- **Shared session core** — extracted `SessionRunner`'s per-session mechanics into `proteus/runtime/_session_core.py` (pure refactor, no behavior change — the existing tests stayed green at the same count). Both `SessionRunner` and the new `InteractiveSession` build their traces from these same helpers.
+- **`InteractiveSession`** (`proteus/runtime/interactive.py`) — a threadless, stepwise driver (`state()`/`step()`/`finish()`) that advances one turn per HTTP request. It is pinned to `SessionRunner(HumanAgent)` by a **golden equivalence test** (`tests/runtime/test_interactive_equivalence.py`) asserting full `model_dump()` equality for the same action sequence — the two paths cannot drift.
+- **Fairness split** (carried from CP5) — the live `state` exposes only the color grid + available actions (no reward/optimal/habit), exactly what the LLM sees; the full per-turn disclosure (your action vs optimal vs habit, reward, 8 metrics) appears in `review` only once the game is over. Asserted in both the `InteractiveSession` and server tests.
+- **Always-saved trace** — every finished game appends a `runs/web_<scenario>_<difficulty>.jsonl` line (override via the `/finish` `{out}` body) that is schema-identical to LLM/CLI traces. Verified end-to-end: a web-produced trace is accepted by both `proteus replay` and `proteus compare` (shows up as a `model=human` baseline group).
+- **HTTP API** (stdlib `http.server`) — `GET /` (page), `GET /config`, `POST /session`, `GET /session/{id}`, `POST /session/{id}/act`, `POST /session/{id}/finish`; a pure socket-free `handle_request(...)` router (unit-tested without a socket) + in-memory registry; structured errors (400/404/409/500); offline import-safety guarded (`tests/web/test_import_safety.py`).
+
 ## How to run
 
 - Environment: `python` is **not** on PATH; a gitignored venv lives at `.venv` (Python 3.12,
   pydantic v2 / numpy / pyyaml / pytest **+ matplotlib** — the latter now required by the viz/png
-  tests). **Full suite:** `.venv/bin/python -m pytest -q` → **103 passed**, no network, no display.
+  tests). **Full suite:** `.venv/bin/python -m pytest -q` → **148 passed**, no network, no display.
   Provider SDKs are NOT installed in `.venv` (keeps the offline invariant structural); matplotlib runs
   **Agg-only** (headless), so it does not break the offline/no-display invariant. Recreate `.venv` with:
   `uv venv --python 3.12 .venv && uv pip install --python .venv/bin/python "pydantic>=2" "numpy>=1.26" "pyyaml>=6" "pytest>=8" "matplotlib>=3.8"`
@@ -138,6 +149,10 @@ Executed task-by-task via subagent-driven-development (3-gate: implement → spe
   predator_evade --seed 42 --play-turns 6 --out runs/h.jsonl` (live view is the same ASCII the LLM sees).
 - Visual replay: `.venv/bin/python -m proteus replay runs/h.jsonl --visual --fps 0` (truecolor terminal)
   or `--png runs/frames` (per-frame PNGs). Plain `replay runs/h.jsonl` stays text-only.
+- Web interactive play (offline, stdlib-only): `.venv/bin/python -m proteus.web` (`--host`/`--port`,
+  defaults `127.0.0.1:8000`), then open the printed `http://127.0.0.1:8000/` — play the color grid in the
+  browser; the finished game appends a schema-identical trace to `runs/web_<scenario>_<difficulty>.jsonl`
+  (replayable/comparable with `proteus replay`/`compare`). Zero new dependencies.
 - Offline CLI smoke: `.venv/bin/python -m proteus run --scenario predator_evade --model fake:demo
   --seed 42 --play-turns 5 --out runs/x.jsonl && .venv/bin/python -m proteus replay runs/x.jsonl`
 - Difficulty bands: add `--difficulty {easy,medium,hard,expert}` to `run`/`play` (EASY is the default,