chore(core): reduce sequencer txn log file storage record length from 60 to 40 bytes

[ideoma]

The new sequencer V2 record can be compacted to 40 bytes instead of 60 without sacrificing too much of usable data range and code complexity.

Proposed layout:

• Structure Version: 40 bits, range: 0-1T
• WAL Id: 32 bits, range: 0-4B (unchanged)
• Segment Txn: 24 bits, range: 0-16M
• Segment Id: 24 bits, range: 0-16M
• Commit Timestamp: 48 bits, stored in milliseconds, range: years 1970-10889
• Min Data Timestamp: 64 bits, microseconds
• Max Data Timestamp: 48 bits, stored as the difference from Min Data Timestamp, microseconds. Range: 3 years from Min Data Timestamp
• Commit Row Count: 40 bits, range: 0-1T