fix(xim): make download cache commits transactional

[Sunrisepeak]

Summary

• reject legacy non-empty caches when HEAD fails; allow offline reuse only for identity-bound sidecar v2 commits
• download every candidate into a unique staging file and commit only after validation
• preserve/restore prior cache content across transfer, cancellation, and commit failures
• serialize cache transactions with cross-platform file locks and recover interrupted backup/staging state
• isolate mirror tests from the user's real XLINGS_HOME

Test plan

• TDD RED: cache admission and transaction hooks were missing
• mcpp test — 10 passed, 0 failed
• mcpp build
• git diff --check

Stack

Depends on #357. Retarget to main after #357 merges.

Refs #356

[Sunrisepeak]

Superseded by #359. After #357 was squash-merged, GitHub kept the stacked merge-base and displayed already-merged build-system files in this PR. #359 is rebuilt from current main with only the issue #356 commits; no history was rewritten.