chains: recognise plain UTC as a timezone in proposed spans

[kevin9327]

chains/spans.py matches named zones with [A-Z]{2,4}T\b, which covers GMT, CEST, EST and the rest, plus utc±N for offsets. Plain UTC doesn't end in T, so it never matches, even though ZONES maps "utc": 0.

>>> propose("The call is on 3 September 2026 at 14:00 UTC.")
[Span(kind='datetime', text='3 September 2026 at 14:00', ..., meta={'zone': None, ...})]
>>> propose("The call is on 3 September 2026 at 14:00 GMT.")   # GMT works
[Span(kind='datetime', text='3 September 2026 at 14:00 GMT', ..., meta={'zone': {'offset_h': 0}, ...}),
 Span(kind='timezone', text='GMT', value={'offset_h': 0}, ...)]

So a UTC time stays naive: it ranks last for prefer="with_zone" slots, no timezone span is offered, and ops can't place it against zoned times.

Change: add utc\b to _ZONE_RE, after the utc±N alternative so UTC+2 still matches whole.

Test: test_propose_plain_utc_is_a_zone covers a plain-UTC datetime next to a UTC+2 one (zones, span text, and timezone spans). It fails on main and passes with this change.

uv run pytest -q tests/test_chains.py   # 25 passed

I also ran the other test files one at a time (backends, calibrate, clef_head, device, fanout, jevcompat_issue16, noul_band, patterns_presets, server, train_write_json, version_lockstep), and they all pass. test_client, test_openjev_compatibility and test_primitives didn't finish on this machine within the time limit, and none of them import chains.