Generator: a hard-killed generate leaves <path>.<guid>.tmp files that no later run removes #413

What

The generation writer's ownership-first write (#412, GEN-E8 Task 2) writes every file as a temp file plus rename, using StagedWrite.InstallAsync. The temp file is named <path>.<guid>.tmp and sits next to its target. An exception runs the finally, which deletes the temp file. A hard kill (SIGKILL, power loss, an OOM kill) skips the finally and leaves the temp file behind.

Why it matters

Nothing removes such a file later: each run writes a temp file with a new GUID name. .gitignore:108 hides it, so stray files pile up unseen under src/OpenCode.Sdk/. They are invisible to git status and to generate --verify. That is harmless to correctness, because the next run still repairs every owned path, but it is unbounded litter in the source tree.

Proposed fix

At the start of a write, after validation and before step 2, delete every *.tmp file next to an owned path whose name matches the writer's <owned path>.<guid>.tmp shape. The writer owns those names and nothing else creates them. Add a test row: a stray A.cs.<guid>.tmp from a killed run is gone after the next run, and an unrelated *.tmp is untouched.

Found at the coordinator's and the lead's review of #412 (2026-10-17). It is a small follow-up for the U7 sweep.