feat(compactor): allow overriding header timestamp

[corylanou]

Summary

• add optional Compactor.HeaderTimestamp override for the output header timestamp
• keep default behavior when the override is unset
• add test coverage for the timestamp override

Why

Compacted L1+ files currently inherit the latest input timestamp. For PITR, downstream tools need the earliest timestamp preserved after L0 retention.

Testing

• go test ./...

Links

• Closes Compactor: allow preserving earliest input timestamp for PITR #74
• Refs PITR fails after L0 retention expires, even when data exists in compacted files benbjohnson/litestream#1038

[corylanou]

Closing based on Ben’s guidance: compacted LTX files represent the latest state only, so their header timestamp should remain the last input timestamp. Earliest timestamps aren’t valid PITR targets once L0 files are compacted, so this change would be misleading.