test(brain): canary 7 — local/Ollama parity, and the seam that makes it possible

[AetherAI3]

The last of the seven release canaries, and the one that stayed unwritable
longest. LocalBrain spawned a Python module with no injectable transport, so
nothing in this repo could drive it: a search for new LocalBrain across test/
returned nothing, because it was not constructible under test.

Adds a BrainSpawner seam. LocalBrain already speaks a newline-delimited JSON
protocol over stdio, so a fake that speaks the same protocol is indistinguishable
from the real child. Default behaviour is unchanged — the option simply defaults
to node's spawn.

What parity means here is deliberately NOT "identical events". Ollama is a
pure-TypeScript loop; the Python path has a far richer vocabulary (stage, skill,
telemetry, checkpoint). Requiring identical output would be a false contract.

It means a host driving either brain sees the same SHAPE: the same tool calls in
the same order, each carrying an id it can reply to, terminated by exactly one
done. A host loop must not need to know which brain it is talking to. The test
normalizes both transcripts to that view and requires them equal.

Four assertions:

both brains present the same shape to a host loop
every tool_call from either brain carries a replyable id
each brain terminates with exactly one done
a brain that dies without a done still ends its stream

The last one guards a hang rather than a wrong value: a host awaiting a terminal
event that never arrives waits forever.

Worth recording, because it nearly produced a false finding: the first run of
that fourth test hung for ten minutes. The cause was the fake, not the product.
LocalBrain ends its stream on the child's close event, and the fake's kill()
closed stdout without emitting close — something a real child always does. A
fake that is unfaithful about its termination signal turns a correct product
into a hanging test. Fixed in the fake; the product path was right.

Gates at this commit:
npm run typecheck exit 0
npm test 1117 pass / 0 fail

All seven canaries now run on every build:

1 denied mutation test/release_canaries.test.ts
2 live-child cancel test/process_tree.test.ts
3 reconnect replay test/release_canaries.test.ts
4 remote freshness test/worktree.test.ts
5 fake-gh ship test/ship_rail.test.ts
6 cap across reconnect test/release_canaries.test.ts
7 brain parity HERE