event_epoch() and `X or Y` fallbacks treat epoch 0 (and other zero integers) as missing

[erskingardner]

Impact

Epoch 0 is a legitimate, common value (MLS groups start at the genesis epoch 0), and value_if_int() explicitly accepts 0 as valid. But the analysis layer collapses falsy integers with X or Y chains, so a real epoch of 0 is treated as "absent". This produces incorrect forensic output: genesis-epoch events are mis-bucketed in the timeline, and 0 renders as – in summaries.

event_epoch() is the worst case because its result is used to bucket message events per epoch and to set each timeline item's epoch:

def event_epoch(event):
    return (
        event.epoch
        or event.source_epoch
        or event.to_epoch
        or event.pending_epoch
        or event.current_tip_epoch
        or event.selected_tip_epoch
    )

If event.epoch == 0, the or chain skips it and returns the next truthy field (or None), so the event is attributed to the wrong epoch — or dropped from message_event_count entirely.

The same falsy-zero bug appears in several display strings, and in the template via the default filter (which also treats 0 as empty).

Code pointers

• forensics/analysis.py:433 — event_epoch() or chain drops epoch 0.
• forensics/analysis.py:720 — message_counts[event_epoch(event)] mis-buckets/loses epoch-0 message events.
• forensics/analysis.py:795 — timeline item "epoch": event_epoch(event).
• forensics/analysis.py:399 — selected_tip_epoch or '-' renders epoch 0 as -.
• forensics/analysis.py:410 — event.epoch or '-'.
• forensics/analysis.py:467 — f"epoch {event.from_epoch or '-'} -> {event.to_epoch or '-'}".
• forensics/templates/forensics/audit_file_detail.html:102 — {{ event.seq|default:"–" }} renders seq == 0 as – (should be default_if_none).

Expected behavior

A stored integer of 0 (epoch, seq, etc.) should be treated as a real value: epoch-0 events should bucket under epoch 0, and 0 should display as 0, not –.

Suggested fix

• Replace the or chains used to pick integer values with explicit None checks, e.g. a small helper first_not_none(*values) for event_epoch().
• Use is not None (not truthiness) wherever a fallback for an integer field is rendered.
• In templates, use default_if_none instead of default for integer fields like seq/wall_time_ms/epochs.
• Add a regression test with an epoch_confirmed/message event at epoch 0 asserting it buckets under epoch 0 and renders as 0.