Append records with segment rolling by size

[mairinkdev]

Summary

Implement basic append: write full record frames (header + payload + checksum) to the current segment. When the segment reaches a configured max size (segment_max_bytes), roll to a new segment. Support sequential scan to read back appended records (no index yet).

Scope

• Log::append(payload) (or equivalent): encode record (header + payload + checksum), append to current segment file.
• Segment roll: when current segment size would exceed segment_max_bytes, close current segment and open next (base_offset = next offset). Configurable segment_max_bytes.
• Sequential scan: iterate records from a given segment (or from first segment) in order; decode and yield (offset, payload) or error on corruption (checksum validation can be Day 5; here decoding is enough).
• Tests: append N records; scan and assert order and content; append until roll, then scan across segment boundary and assert continuity of offsets and payloads.

Acceptance criteria

• Append is append-only; offsets are monotonic across segment rolls.
• Scan reads back exactly what was appended, in order.
• Segment roll triggers at configured size; no record is split across segments.

Suggested commit

feat: append records with segment rolling