Use chrono instead of hand-rolled UTC conversion (#1116)

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

Replaces the hand-rolled civil_from_days / civil copies and the hand-written
weekday / leap-year / days-in-month helpers with chrono. chrono is already a
workspace dependency (used by crates/text), so this reuses that entry and just
adds `chrono = { workspace = true }` to the engine and desktop app rather than
a second version.

- apps/deckcraft logging `timestamp` now formats through chrono (private
  civil_from_days deleted).
- engine cmd::design `civil` (days since 1970 -> date, used for comment
  timestamps in review.rs) uses chrono::NaiveDate.
- text datetime `DateTime::is_valid` / `weekday` use chrono::NaiveDate instead
  of a hand-rolled leap-year table and days-since-1970 weekday.

Output is byte-identical and pinned by the existing logging, design and
datetime-field tests; each converted helper keeps a proleptic-Gregorian
fallback for dates outside chrono's NaiveDate range.

Follow-up to storytold/cadcraft#1785

* text: make chrono available on wasm32

is_valid/weekday in crates/text/src/datetime.rs use chrono unconditionally,
but chrono was only declared under the non-wasm target, so the crate (and
engine/pdf/render/ui-egui depending on it) failed the wasm32-unknown-unknown
check in `cargo xtask ci`.

- crates/text: declare chrono unconditionally (wasm-safe `std` baseline),
  and keep `clock` in a separate cfg(not(target_arch="wasm32")) entry for
  now_local()'s local clock, which is already wasm-gated to return None.
- workspace: drop `clock` from the shared chrono entry so it is no longer
  forced onto wasm builds (engine uses chrono unconditionally too); `clock`
  is now requested per-crate only where the local clock is read.

* Never panic on a clock outside chrono's range; harden the civil fallback

logging.rs timestamp(): chrono's From<SystemTime> unwraps, so a clock past
year 262143 panicked inside the logger. Use the fallible DateTime::from_timestamp
and read an out-of-range or pre-epoch clock as the epoch, as the old saturating
code did. Years past 9999 are written with {:04} like before, because %Y adds a
'+'. Tests cover both cases.

cmd/design.rs civil_fallback(): make the hand-rolled fallback's year arithmetic
saturating so civil() cannot overflow or panic at i64::MAX/MIN (the engine rule
forbids crashes and unchecked arithmetic). Behaviour is unchanged for every real
input; a test exercises both extremes. The output year is formatted with {y:04}
by review.rs, so no %Y prefix issue.

crates/text/src/datetime.rs: checked after review; is_valid already limited the
year to 1..=9999 before this PR, weekday() is only reached for valid dates, and
all its arithmetic is checked/euclidean, so no change is needed.

* 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.

* Make civil() exact for every i64 instead of saturating

The out-of-range fallback saturated z + 719_468, so civil(i64::MAX)
returned a date 719,468 days early. civil() now has chrono convert the day
within its 400-year Gregorian cycle and adds whole cycles back as years
(i128 for the extremes), so the hand-rolled fallback is gone. Exact results
for i64::MAX/MIN and both sides of chrono's range are tested against Hinnant's formula.

* time: preserve out-of-range years in the log timestamp instead of the epoch

`DateTime::from_timestamp(..).unwrap_or_default()` changed the existing output: a
clock set far ahead (an input the old hand-rolled civil-from-days converted)
collapsed to `1970-01-01`. Restore the old conversion domain with a cycle-based
helper: reduce by whole 400-year Gregorian cycles (146_097 days), convert the
in-range remainder with chrono, and carry the year back as i128. Only the year
leaves chrono's range; month/day/time stay exact. Pre-1970 still clamps to the
epoch as before. chrono stays the only calendar logic (no new features).

apps/deckcraft logging.rs `timestamp` (SystemTime, u64 secs), pinned via a
platform-safe `UNIX_EPOCH.checked_add`:
  u64::MAX/2 s → "292277026596-12-04T15:30:07.000Z" (was "1970-01-01..."),
  10e12 s → "318857-05-20T17:46:40.000Z", 253_402_300_800 s → "10000-01-01...".

* 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.

crates/engine/src/cmd/design.rs

@@ -943,18 +943,21 @@ pub fn date_field_text(kind: &str) -> String {
     DateTime::now_local().and_then(|now| field_text(kind, None, now)).unwrap_or_default()
 }
 
-/// Days since 1970-01-01 → (year, month, day) (Howard Hinnant's algorithm).
+/// Days since 1970-01-01 → (year, month, day), exact for every `i64`.
 pub fn civil(z: i64) -> (i64, u32, u32) {
-    let z = z + 719_468;
-    let era = z.div_euclid(146_097);
-    let doe = z.rem_euclid(146_097);
-    let yoe = (doe - doe / 1460 + doe / 36_524 - doe / 146_096) / 365;
-    let y = yoe + era * 400;
-    let doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
-    let mp = (5 * doy + 2) / 153;
-    let d = (doy - (153 * mp + 2) / 5 + 1) as u32;
-    let m = if mp < 10 { mp + 3 } else { mp - 9 } as u32;
-    (if m <= 2 { y + 1 } else { y }, m, d)
+    use chrono::Datelike;
+    // `NaiveDate` covers only about ±262143 years, and the Gregorian calendar repeats every 400
+    // years (146_097 days). chrono converts the day within its cycle and whole cycles go back
+    // into the year (an i128 for i64::MAX/MIN). 1970-01-01 is `num_days_from_ce` 719_163, and
+    // 0..146_097 days from it always fits.
+    let (cycles, rest) = (z.div_euclid(146_097), z.rem_euclid(146_097));
+    match i32::try_from(rest + 719_163).ok().and_then(chrono::NaiveDate::from_num_days_from_ce_opt) {
+        Some(d) => {
+            let y = i128::from(d.year()) + i128::from(cycles) * 400;
+            (i64::try_from(y).unwrap_or(if y < 0 { i64::MIN } else { i64::MAX }), d.month(), d.day())
+        }
+        None => (1970, 1, 1),
+    }
 }
 
 fn master_view(s: &mut Session, p: &Value) -> Result<Value> {
@@ -1495,6 +1498,11 @@ mod tests {
     fn civil_dates() {
         assert_eq!(super::civil(0), (1970, 1, 1));
         assert_eq!(super::civil(20_734), (2026, 10, 8));
+        // Past `NaiveDate`'s range the 400-year cycles carry the year, exactly, up to the extremes.
+        assert_eq!(super::civil(95_026_602), (262_144, 1, 1));
+        assert_eq!(super::civil(-96_465_964), (-262_145, 3, 1));
+        assert_eq!(super::civil(i64::MAX), (25_252_734_927_768_524, 7, 27));
+        assert_eq!(super::civil(i64::MIN), (-25_252_734_927_764_585, 6, 7));
     }
 
     #[test]