Avoid ftruncate during ledger rollback

[Copilot]

Closes #7837.

Ledger rollback used ftruncate() on the hot path, which is expensive on Azure Files. Committed ledger files still need canonical sizes across nodes, so physical truncation is deferred until chunk completion.

• Rollback path

  • Replaces immediate physical shrink with a truncation marker written at the logical end of the ledger file.
  • Recovery treats the marker as an invalid oversized entry and stops before stale tail data.

• Completion path

  • Performs the eventual ftruncate() when completing the ledger chunk.
  • Preserves canonical completed/committed file sizes.

• Coverage

  • Adds ledger unit coverage for deferred physical shrinking, recovery after marker writes, and eventual shrink before commit.

// Rollback now records the logical end without shrinking the file immediately.
write_truncation_marker(physical_size);

// Completion later canonicalizes the file size.
truncate_physical_file(completed_file_size);