Fix race condition when two processes build the same core image

[kisp]

Fixes #3

What was wrong

ensureImage checks doesFileExist, gets False, and calls makeImage. If two processes do this simultaneously they both pipe a Lisp script ending in (sb-ext:save-lisp-and-die "<imagePath>") to separate SBCL processes. Both write to the same path concurrently, corrupting the core file.

Fix

Each call to makeImage now writes to a private temp file:

<imagePath>.tmp.<pid>

On success, POSIX rename(2) moves it atomically to imagePath. rename on Linux replaces the destination if it already exists, so if two processes race to rename, one silently overwrites the other with an equivalent image and both callers proceed normally.

On build failure the temp file is removed (best-effort via catchIOError).

Changes

• sbclScript gains a tmpPath parameter; save-lisp-and-die targets tmpPath while ensure-directories-exist still uses imagePath (same directory, ensures the cache dir exists)
• makeImage calls getProcessID to form tmpPath, passes it to sbclScript, then renames on success or cleans up on failure
• New imports: getProcessID, System.Posix.Files.rename, catchIOError

Test plan

• nix build passes (binary produced)
• HOME="$(mktemp -d)" nix run .#test to run the integration suite