Apply a bounded patch atomically in memory

Apply edits to a virtual repository mapping. Each edit supplies a relative POSIX path, nonempty old text, and replacement new text. Only existing files are supported, with at most one edit per path. The old text must occur exactly once in the original file. Reject absolute paths, empty/dot/traversal segments, backslashes, stale context, and ambiguity. Validate all edits before returning a new mapping. Never mutate inputs or execute the edited code. This is an educational patch format, not a Git patch parser or a security sandbox.