Make the page playable: interactive board, live solver, and a usable run picker

Three things the build was quietly missing.

The interactive board was never mounted. The contract has `interactive.init` and
`interactive.Controls`, the demo implemented both, and the shell's `split-play`
beat rendered only the replay — so the beat titled "you and the model get the
same word" showed one board. PlayYourself now renders the visitor's attempt from
the same seed as the run beside it, generically: it knows only the contract, so
any demo shipping an interactive mode gets it and one that does not renders
nothing rather than an empty pane.

solver.worker.ts was dead code — nothing constructed it, which is how CI caught
it: `new Worker(` appeared nowhere in the bundle. It is wired now behind "what
would the best player guess?", and it answers in 92ms from a real worker on
boards no recording covers. That is the difference between a demo and a video.
It also surfaces the moment the solver picks a word that CANNOT win, which is
the counterweight visible in one line instead of explained in a paragraph.

The CI check that found it was itself wrong: it grepped every bundled file for
`blob:`, which React's own code contains in a scheme check, so it failed on a
risk that was not present. It now greps for worker construction from a blob,
which is the thing production CSP actually blocks in silence.

And the run switcher was thirty buttons carrying four distinct labels. Split
into arm and seed, holding the seed across an arm change — comparing two agents
means comparing them on the same hidden word, and silently jumping seeds would
break that while looking fine.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019mt6sHQHEnEYrJZvoMCJSB

envs/build_manifest.py

@@ -17,6 +17,7 @@ ARMS = {
     "base-off": ("Out of the box", "recorded"),
     "base-on": ("Allowed to think", "intervened"),
     "solver": ("Best-known play", "generated"),
+    "cautious": ("Never wastes a guess", "generated"),
 }
 
 # What was done to the run, for arms that had something done to them. Required
@@ -26,7 +27,7 @@ INTERVENTIONS = {
     "base-on": "Same model, same seeds, sampled with thinking enabled. No training, no fine-tuning.",
 }
 
-ORDER = ["base-off", "base-on", "solver"]
+ORDER = ["base-off", "base-on", "solver", "cautious"]
 
 
 def main() -> int: