Use chrono instead of hand-rolled UTC conversion (#2457)

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

Replaces the hand-rolled Howard Hinnant civil/days-from-civil copies and the
leap-year/days-in-month table with chrono as a workspace dependency:
- apps/lightcraft/src/logging.rs: timestamp (log line time)
- crates/catalog/src/dates.rs: civil + weekday
- crates/catalog/src/stacks.rs: iso_seconds (date -> seconds)
- crates/engine/src/cmd/face_recognize.rs: capture_secs (native only)
- crates/meta/src/datetime.rs: DateTime::unix_seconds and the valid() day check
Output formats and semantics are unchanged (pre-epoch clamp, day rollover that
normalize_iso relies on, leap-second tolerance, EXIF/XMP parsing). chrono uses
only std, so wasm stays safe. AGENTS.md/CLAUDE.md now say 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, so a clock past year 262143 panicked
inside the logger. timestamp now uses the fallible DateTime::from_timestamp
and reads such a clock as the epoch, keeping the old pre-epoch clamp.

timestamp and catalog's civil write years past 9999 with {:04} like the old
{y:04} did, because %Y would add a '+'. Tests cover the out-of-range and
5-digit-year cases.

* 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/lightcraft/src/logging.rs: `timestamp` via `utc_parts`; keep the
  pre-1970 epoch clamp; u64 secs past i64::MAX saturate, not epoch.
- crates/catalog/src/dates.rs: `civil` cycle-based (the callers stay inside
  0000-9999, but a far clock no longer resets to the epoch).

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
  civil 10_000_000_000_000 -> 318857-05-20T17:46:40,
        -10_000_000_000_000 -> -314918-08-13T06:13:20, i64 extremes

* 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

@@ -62,6 +62,7 @@ of it. Full standard: `../craftrules/standards/never-crash.md`
 - **Layering** (`plan/architecture.md` §3, enforced by `cargo xtask layers`): nothing below L5 depends on egui/eframe/winit/rfd.
 - **Everything is a command** (`crates/engine`): id, label, menu path, shortcut, params, enabled(), run(). UI, CLI, control channel and MCP all dispatch by id. Every slider is a `develop` control spec.
 - **Resolution independence:** settings use normalized image coordinates and relative radii; previews and exports must match.
+- **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).
 - **Quality gates** before every commit: `cargo xtask ci` (fmt, clippy -D warnings, tests, layers, assets, wasm).
 - **Commits:** one task id per commit (`M2.3: local Laplacian highlights/shadows`). Only green states. End messages with the attribution line required by the environment.