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 and contents across nodes, so physical truncation is deferred until chunk completion.

• Rollback path
  • Replaces immediate physical shrinking with a marker at the logical end of the ledger file.
  • Keeps the marker ahead of subsequent writes, including non-committable writes, so a restart cannot recover stale tail entries.
• Recovery path
  • Recognises the marker as an intentional logical end rather than malformed ledger data.
  • Preserves the durable incomplete-file transition when replaying existing completed chunks.
• Completion path
  • Performs the eventual ftruncate() before publishing the positions table offset.
  • Produces canonical completed and committed file bytes.
• Coverage
  • Covers deferred physical shrinking, graceful and abrupt restart recovery, marker movement after new writes, and byte-for-byte equivalence with a clean committed ledger.

[Copilot]

Pull request overview

This PR addresses #7837 by removing ftruncate() from the ledger rollback hot path (notably expensive on Azure Files) while preserving the invariant that completed/committed ledger chunk files are byte-identical across nodes. It introduces a “logical truncation” marker to prevent recovery from replaying stale tail bytes, and defers the eventual physical truncation to chunk completion time.

Changes:

• Add a truncation-marker mechanism in asynchost::LedgerFile to represent a logical rollback without immediately shrinking the file.
• Teach incomplete-ledger recovery to treat the marker as an intentional logical EOF and track deferred physical truncation state.
• Add a host-ledger unit test covering deferred shrink behavior, crash/restart recovery behavior, and byte-for-byte equivalence with a canonical committed ledger.

Custom instructions used:

• .github/copilot-instructions.md
• .github/instructions/reviewing.instructions.md

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/host/ledger.h	Implements deferred truncation via a truncation marker during rollback/rewrite, recognizes marker during recovery, and performs physical truncation during completion.
src/host/test/ledger.cpp	Adds a regression test validating deferred physical shrink, crash recovery behavior, and equivalence of committed outputs versus a canonical baseline.