Fix overlay ctime stat cache

[elithrar]

Supersedes #17 by preserving Git's ctime stat-cache guard for writable overlay files and hardening the related overlay rename paths.

The original issue was that overlay metadata let caller-controlled mtime stand in for ctime, so Git could miss dirty files after same-size rewrites that restored mtime. This keeps ctime independent and makes rename/reconcile behavior preserve that invariant instead of creating stale overlay state.

• Preserve separate overlay ctime for writes, truncates, mtime updates, and renames.
• Keep tracked rename metadata coherent across chained renames, HEAD changes, and stale overlay reconciliation.
• Preserve untracked create renames, while treating create/symlink renames over existing base files as local modifications.
• Reject unsupported file/directory rename combinations instead of creating overlay entries that shadow the wrong base type.
• Clean up stale rename whiteouts and directory tombstones safely, including wildcard and UTF-8 path cases.

Tests: go build ./cmd/artifact-fs; go vet ./...; go test ./...; AFS_RUN_E2E_TESTS=1 go test -run '^TestE2EGitStatusDetectsSameSizeRewriteAfterMtimeRestore$|^TestE2EGitCommitTrackedRename$|^TestE2E$' -v .