Recover M3 WIP boot image when boot.bin is missing

[pkyanam]

When an experimental Asahi kernel leaves boot.bin missing while boot.bin.old or boot.bin.omarchy-pre-wip remains, omarchy-mac-kernel-wip --rollback-boot cannot discover the ESP path and refuses to restore the saved image. The installer also continued after failing to create its recovery snapshot.

This follow-up makes the recovery path functional for that state:

• discover an ESP boot.bin path from either saved image;
• copy the saved image to a fresh same-directory temporary file and rename it into place, so a failed copy or rename does not truncate the active image;
• refuse the WIP package transaction when the pre-install snapshot cannot be created.

The regression test uses the real helper functions and a disposable ESP-shaped directory. Against PR343 baseline ea3267b, the saved-image-only rollback case fails. At c22ff171a7be257fbda7d2176d322a52bbb4af7b, it restores successfully; the test also covers stale restore directories, failed copy/rename cleanup, package-install refusal on snapshot failure, and the existing rollback/remove cases.

Validation: bash test/shell.d/apple-kernel-wip-test.sh, bash -n on the command and test, and git diff --check pass. This was validated with scratch files on macOS/APFS. No native Linux ESP, package-manager transaction, reboot, firmware update, or physical M3 boot was performed, so those runtime paths remain for maintainer validation.

This change was AI-assisted and independently QA-tested and reviewed. Related M3 integration context: omacom#343