perf(people): align response recorded_at precision with canonical People serializer #390

Finding

While validating #389's pre-parse response byte ceiling, the current #317 success-response timestamp contract exposed a contradictory unbounded field: recorded_at accepted arbitrary fractional-second precision (\.\d+) even though canonical People #64 serializes EmploymentSeparationResult.recorded_at with Python datetime.isoformat(), which emits no fractional component when microseconds are zero and otherwise emits at most six fractional digits.

That made two things wrong at once:

• a governed-looking 200 response with seven or arbitrarily many fractional digits could be accepted as commercial evidence even though the canonical owner cannot emit it;
• because that field was semantically unbounded, no finite pre-parse byte budget could honestly preserve every canonical-owner-representable response admitted by the validator.

This is a buyer-evidence false-GREEN and the prerequisite that makes #389's bounded response representation coherent. It supersedes only the earlier response-side assumption that arbitrary fractional precision should be accepted; result/runtime/fixture chronology precision from #363–#366 remains unchanged.

Repair lineage

• Test-first 790468c5fa174cd0b045226ab88ffb846dd6b30a adds a dedicated response timestamp precision regression. Canonical no-fraction, one-digit and six-digit values must pass; seven digits and an intentionally huge fractional component must fail.
• Causal repair f8d8eb061c1be2432904d342894798e04fe57255 narrows only the HTTP success-response timestamp lexical contract to 0..6 fractional digits. Gregorian year/month/day/clock validation, UTC Z, the closed success envelope, operational UUID/replay checks, perf(people): bound governed response JSON before strict parsing #389 byte budget and all result/runtime/fixture timestamp logic are unchanged.
• Focused perf(people): bound governed response JSON before strict parsing #389/perf(people): align response recorded_at precision with canonical People serializer #390 boundary reproduction is 7/7 GREEN locally. This is developer-side boundary evidence only; there is no hosted exact-head k6 run and no commercial latency result is claimed.

Keep this issue open through protected/integration acceptance; branch-local repair is not completion.

Owner: Draft #317 (perf/employment-separation-buyer-path).