as-of filter string-compares two different timestamp formats

[jpwinans]

theloom/viz/temporal.py:15 filters events with at > as_of, where at is Event.timestamp — a datetime.isoformat() string (…816000+00:00) — while asOf arrives in the wire format (…816Z). Two different shapes, compared as strings.

The codebase already has the guard against exactly this: _entry_id_to_wire_iso (theloom/store/events.py:247), and entry_id_timestamp's docstring (events.py:208-214) explains why cross-shape comparisons must use the wire form.

Today the mismatch is benign by accident: when prefixes tie through the millisecond digits, '0' < 'Z' errs toward inclusion, which is the harmless direction for an as-of read. But it is the precise hazard the helper exists to prevent, and any future consumer comparing the other direction inherits it silently.

Fix: normalize both sides to the wire shape at the comparison (or render Event.timestamp in wire form at construction — check blast-radius on other Event.timestamp consumers first). Pin with a test whose event and pivot tie through the millisecond.

(Found while diagnosing the flaky test_as_of_truncates_temporal_events, fixed separately in PR #44 — that flake was clock-domain skew; this is the adjacent format inconsistency.)

[jpwinans]

Provenance note (from a parallel filing, #52, now closed as dup): surfaced during PR #44 acceptance while deflaking test_as_of_truncates_temporal_events. That flake had a separate clock-skew cause (client vs server clock across a 10ms guard band), already fixed in f401c5b by deriving the pivot from server-side event timestamps. This issue is the latent format-mismatch underneath — benign today only because when the fraction prefixes tie, '0' < 'Z' errs toward inclusion.