feat: convert Temporal values to and from Python datetime types

[imfing]

Why

The V8 15 upgrade (#10) shipped a mature, Rust-backed Temporal implementation — but any Temporal value crossing into Python arrived as an empty dict {} (its data lives in internal slots, invisible to generic object conversion). Meanwhile Python date/time/timedelta raised Unsupported Python type. This closes both gaps using the package's existing typed-conversion identity.

Mechanism

Temporal values travel as tagged payloads, the same pattern Date/Set/BigInt already use, across all four boundary paths:

JS -> Python:  temporalPrepare(v)  ->  { __jsrun_type: "Temporal", kind, ...fields }
               (bridge prepare() for op args; runner calls the same helper for eval results)
Python -> JS:  date/time/timedelta -> tagged payload -> temporalRevive()
               (bridge revive() for op results; runner calls the helper for call args)

Instant        <-> aware datetime, UTC      (epoch_ns as string; sub-us truncated)
ZonedDateTime  <-> aware datetime           (ZoneInfo, or fixed offset like "+05:30")
PlainDate      <-> date                     (non-ISO calendars normalized via withCalendar)
PlainTime      <-> time
PlainDateTime  <-> naive datetime
Duration       <-> timedelta                (exact; BigInt ns arithmetic)

Blast radius

Area	Impact
ops.rs bridge	+temporalPrepare/temporalRevive helpers (also exposed as globals for the Rust paths); one hook in prepare(), one case in revive()
runner.rs	Eval-result path detects Temporal via the helper; js_value_to_v8 upgrades tagged payloads via the helper
conversion.rs	Temporal tag → Python types; new date/time/timedelta branches (previously an error, so purely additive)
datetime behavior	Unchanged — still maps to JS Date (back-compat)
Not converted	Duration with nonzero years/months/weeks (calendar-relative, no fixed length); aware time raises a clear error
JSValue enum / serde	Untouched — tagged-object flow needs no new variants

Validation

• 21 new tests in tests/test_temporal.py: all six types both directions, ops path, IANA + offset time zones, non-ISO calendars, negative epochs/durations, nesting, round-trips, back-compat.
• Full suite: 321 passed; make lint clean.

[imfing]

Addressed both review findings (44ca27e):

• Large durations: dropped the i64 narrowing — total microseconds now pass as an i128 → arbitrary-precision Python int, with timedelta's own constructor enforcing range. timedelta(days=106751992) round-trips.
• Year-boundary zoned datetimes: the conversion no longer materializes the intermediate UTC datetime. It probes the zone's offset at the instant, then builds local wall-clock fields with integer math (date.fromordinal), so 0001-01-01T00:00:00+01:00[+01:00] and the year-9999 counterpart convert correctly. As a bonus this also fixed DST fall-back folds: ambiguous wall times now carry fold=1 when that occurrence matches the instant's offset (new regression test).

3 new tests; 324 total passing, lint clean.