docs(m174): add spec — deterministic timing assertions

Three tests failed in one afternoon for reasons unrelated to the code under
test: patch_concurrent asserted a client stopwatch for a timeout Postgres
enforces server-side, FleetLibrariesView could not find a dialog it had
rendered correctly, and stream_registry ran 60ms rounds on a loaded runner.
Each looked like a real regression until it was read closely.

Classified all 36 wall-clock assertions across 18 Zig test files plus the
TypeScript surface. They are four different things wearing the same syntax:
17 termination ceilings, 5 timeout floors, 11 stored-expiry value assertions,
and 3 ordering claims written as stopwatches.

So the variable is margin, not the presence of a clock. db/pool_test bounds
15ms of work at 1s and has never flaked; the assertion removed this week
allowed 500ms over a 5s wait. Floors and value assertions are left alone
deliberately — load only makes a floor more true.

Six sections: ledger first, because three classes share one syntax and
converting on syntax would delete sound coverage; the lint last, because it
can only encode a rule the sweep has proven workable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>