perf(web): R.3 — the board stops waiting its turn

[Willi363363]

Third step of track R, carrying its own R.3 ⬜ → ✅ tick. Based on #316,
not on staging: both steps edit the
same twenty lines of readHome, and resolving that in GitHub's interface is the
one thing method/01-git-flow.md forbids. Rebased onto staging and retargeted
as soon as #316 merges.

readBoard was awaited on its own line, and only then did readHome open the
Promise.all below it. Nothing in that group fed the board and the board fed
none of them — same viewerId, same clock. The guest path had it twice over:
the day's tile waited behind the board and needs nothing from it either.

The measurement, including the one that says nothing

On this machine the change does not measure. 40 runs of the guest home
against a local Postgres:

before   4.45 ms median
after    4.81 ms median

Inside the noise — a loopback round trip is smaller than the variance. Recorded
that way rather than dressed up, because 09-query-debt.md's rule is that a
note without a measurement is a guess, not that every measurement must flatter
the change.

What does measure is the depth of the waterfall. 25 ms injected per read,
which is roughly a Neon round trip from a Vercel function:

signed in   50.8 ms → 25.8 ms
guest       50.6 ms → 25.4 ms

Exactly one round trip, on both paths. That is where this pays: not on a
developer's loopback, on the deployment the project actually runs.

The tests assert the structure, not a timing

Every read a home needs is in flight before any of them has answered — which is
the finding stated as an assertion, and does not depend on how fast anything is.
Both cases fail against the old shape.

A promise created before a branch is the shape track Q spent five steps on,
so two more cases say a failing board is a rejected readHome on both paths.
Probed by floating it on purpose: the suite then reports 9 passed, 1 error and
the job fails with no failing case — 10-test-debt.md's own Vitest entry,
arriving from the other side.

Checks

• turbo run test --force --concurrency=1: 10 of 10 tasks, 3 872 passed, 0 skipped.
• turbo run typecheck lint --force: 22 of 22.
• pnpm format:check: green.
• 09-query-debt.md loses its third entry, back at exactly 200 lines.

🤖 Generated with Claude Code

https://claude.ai/code/session_01DsMRxyowr2Ch5RwiAa3t9z

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