docs: record what this session cost to find

Six lessons, each of which took a deployment and a user's patience to locate.
Five are the same shape: something that fails silently and looks like something
else. The asyncio task nobody holds, `proc.wait()` deadlocking on a pipe it will
not read, a peer whose loss is noted but not acted on, a service worker that is
active without controlling the page, and a React setter that outlives its state.

The sixth is about method: `node --check` validates syntax and not names, and
`e2e.py` is a second implementation of the client rather than a test of this
one. Neither caught any of the above. Source-reading tests are weak evidence and
are the only evidence available for the SPA — so prefer ones that re-derive a
value from the source over ones that restate it.

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

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