Fix overlay ctime stat cache

[elithrar]

Builds on Richard Morrill's timestamp fix in #17 by preserving Git's ctime stat-cache guard for writable overlay files.

Case	Before	After
Base attrs	Missing atime/ctime could produce invalid zero timestamps	Base attrs use stable commit time for atime/mtime/ctime
Overlay attrs	ctime effectively followed caller-controlled mtime	Overlay stores and reports ctime separately from mtime
Same-size rewrite + mtime restore	Git could miss the dirty file	Git still sees changed ctime and reports it dirty
Truncate	Size/time metadata could drift from backing file changes	Truncate updates overlay size, mtime, and ctime together

Tests: go build ./cmd/artifact-fs; go vet ./...; go test ./...; AFS_RUN_E2E_TESTS=1 go test -run TestE2EGitStatusDetectsSameSizeRewriteAfterMtimeRestore -v .