Use chrono instead of hand-rolled UTC conversion (#8234)

* Use chrono for UTC/calendar conversion instead of hand-rolled copies

Replaces the civil_from_days copies behind the app's log timestamp, the
Measurement Log's date/time, and the perf report date with chrono as a
workspace dependency (reusing the version already in the tree). Output formats
are unchanged and pinned by the existing tests. AGENTS.md now says time goes
through chrono.

Follow-up to storytold/cadcraft#1785

* Never panic on a clock outside chrono's range; keep 5-digit years unprefixed

chrono's From<SystemTime> unwraps internally, so a clock past year 262143
panicked. logging::timestamp and analysis_cmds::now_iso now read the duration
since the epoch and go through the fallible DateTime::from_timestamp, reading
an out-of-range or pre-epoch clock as the epoch as the old saturating code
did. All three ISO helpers (timestamp, now_iso, perf::iso_utc) write the year
with {:04} rather than %Y, which prefixes a '+' past year 9999.

Tests cover the far-future and year-10000 cases for timestamp and iso_utc.

* Don't build an unrepresentable SystemTime in the far-future test

UNIX_EPOCH + Duration::from_secs(u64::MAX / 2) overflows Windows'
SystemTime and panicked before timestamp() ran, failing the Windows
test job. Build it with checked_add and only assert where it exists.

* time: preserve old out-of-range date conversion (no epoch fallback)

`DateTime::from_timestamp(...).unwrap_or_default()` silently reset any
instant past chrono's year range to 1970-01-01, changing output the old
hand-rolled Howard-Hinnant code converted exactly (e.g. 10_000_000_000_000 s
was 318857-05-20 17:46:40, not the epoch). Restore the old conversion domain
with a cycle-based helper that routes the calendar through chrono (the
calendar repeats every 400 years = 146_097 days, so month/day/time come from
chrono and only the year is carried in i128, keeping every i64 exact).

- apps/photocraft/src/logging.rs: `timestamp` via `utc_parts`; keep the
  pre-1970 epoch clamp; u64 secs past i64::MAX saturate, not epoch.
- crates/engine/src/analysis_cmds.rs: extract pure `iso_from_unix` (cycle
  based) from `now_iso` so the conversion is tested directly.
- xtask/src/perf.rs: `iso_utc` cycle-based, accepts the full u64 range the
  old code did.

Pinned old outputs in tests:
  logging u64::MAX/2 s -> 292277026596-12-04T15:30:07.000Z (was epoch),
          10_000_000_000_000 s -> 318857-05-20T17:46:40.000Z
  iso_from_unix 10_000_000_000_000 -> 318857-05-20T17:46:40Z,
          -10_000_000_000_000 -> -314918-08-13T06:13:20Z, i64 extremes
  iso_utc u64::MAX -> 584554051223-11-09T07:00:15Z (was epoch),
          10_000_000_000_000 -> 318857-05-20T17:46:40Z

* Don't build a year-318857 SystemTime unchecked in the logging test

Windows' SystemTime only reaches about year 30828, so
UNIX_EPOCH + Duration::from_secs(10_000_000_000_000) panicked before
timestamp() ran and failed the Windows test job. Build it with
checked_add and assert where the platform can hold it. The year-10000
case fits everywhere and stays unconditional.

* Name the 400-year range extension as the one exception to the time rule

The rule said never to hand-roll calendar math, but the range fix
(shifting by whole 400-year Gregorian cycles so dates past chrono's year
range keep their old output) is cycle arithmetic, so the rule and the code
disagreed. The rule now names that exception: the library still computes
month, day and time, and only whole cycles are carried in the year.

* Say the library computes month and day; time-of-day is plain arithmetic

Narrows the range-extension wording: the date library computes the
calendar fields, and splitting seconds into hours, minutes and seconds
is ordinary arithmetic, which is what the code does.

AGENTS.md

@@ -70,6 +70,7 @@ People trust PhotoCraft with their work, and a crash loses it. A malformed file,
 8. **Performance is a feature.** Benchmark heavy operations on a 24–36 MP image in release. Work per tile in parallel (rayon), skip empty tiles, never scan a full surface per frame (cache per revision), and record before/after timings in the dev log.
 
 9. **Never panic on input** (see *Never crash* above). A command's `run` closure and anything it calls must return `Err`, never panic, for *any* params or document state: validate params, check bounds before indexing or dividing, and reject absurd sizes before allocating. The `panic_hunt` integration test fuzzes every command with adversarial params and must stay green.
+10. **Time goes through `chrono`** (workspace dependency). Never hand-roll UTC/calendar conversion (days-to-date algorithms, leap-year math); a crate owns only its output format string. The one exception is range extension: past the years chrono can represent, shift the date by whole 400-year Gregorian cycles (146,097 days, after which the calendar repeats), let chrono compute the month and day, and add the cycles back to the year (splitting seconds into hours, minutes and seconds is plain arithmetic).
 
 ## 4. Picking work