chore: Record tracing durations via a canonical Duration attribute encoding

[sergerad]

Summary

Adds a canonical tracing encoding for Duration values.

The tracing field registry had no RecordAttribute implementation for Duration, so every call site that records a duration hand-converted it with .as_millis() as u64 (or a saturating try_from in one place) — eight sites across five crates, with inconsistent overflow behavior between them.

This PR adds impl RecordAttribute for Duration: durations are recorded as whole milliseconds in a u64, saturating at u64::MAX. The allowed field names (retry.delay_ms, shutdown.grace_period_ms, snapshots.oldest_superseded_for_ms, timeout.ms) move from NUMBER_FIELD_NAMES onto the impl, so each registry name has exactly one canonical type. All call sites now pass the Duration directly and the manual conversions are deleted.

Note on Option<Duration>: the write-worker site records oldest_superseded_for.unwrap_or_default() rather than the Option itself, because the blanket Option<T> attribute encoding omits the field entirely on None — the field must always be present on the span.

Changelog

changelog = "none"
reason    = "Internal change only."