Mark generated files and detect edits by hash

Problem/Motivation

The decision taken for this module is that hand edits do not live in generated files at all, and that losing one to a regeneration must be impossible rather than unlikely. That guarantee rests entirely on a marker and a content hash written into every generated file.

Proposed resolution

• Emit the @myrestGenerated docblock with the generator version, the blueprint id, the timestamp and the content hash.
• Compute the hash over the file without the hash line itself, so that writing it does not invalidate it.
• Implement the full decision table: no file — write; hash matches and nothing changed — skip; hash matches and the model changed — overwrite; hash does not match — do not write, raise a conflict; no marker at all — never touch.
• A test per row of that table, including the case that matters most: a file edited by hand is not overwritten.

Remaining tasks

Everything.