feat(bench): live observer page

Watch an agent play in real time, replayed with the main game's renderer and
animations. The board holds the stage with the live panel beside it: a
millisecond stopwatch, how long the current step has been thinking, steps,
decisions, pieces cleared, dead taps and rotations, then the agent's running
commentary. A banner over the board names who is playing, self-reported by the
agent via --agent.

The page polls unconditionally; the checkbox only decides whether the cursor
chases the newest step, and it follows the agent into its next run once this
one ends. Clicking any step in the feed replays the board at that moment, and
the arrow keys walk it — there is no scrub bar, since the feed already is one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.claude/launch.json

@@ -0,0 +1,14 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "dev",
+      "runtimeExecutable": "npm",
+      "runtimeArgs": [
+        "run",
+        "dev"
+      ],
+      "port": 3000
+    }
+  ]
+}