feat: add safe standard library I/O

[9Ashwin]

Implements Issue #2 with a standalone internal/library layer for the existing standard v1 manifest and emoticons directory.

• validates bounded manifests, duplicate JSON keys, UTF-8, paths, metadata, image signatures, MD5 and SHA-256
• prevents corrupt manifest replacement and symlink escapes
• adds cancelable cross-process locks, atomic manifest replacement, and committed-state reporting for post-commit errors
• adds concurrent, cancellation, integrity, symlink and fault-injection tests

Validation: go test ./..., go test -race ./..., go vet ./..., golangci-lint, and Windows amd64 cross-build.