fix(build): build privately and swap generated/ after validation

Build into a per-run sibling directory and replace generated/ only after
validation, with a lock around the swap, so a failed or parallel build never
leaves generated/ partial.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -1,4 +1,7 @@
 generated/
+generated.build-*/
+generated.old-*/
+generated.lock
 .superpowers/
 *.log
 *.tmp