[P2] Target install cannot restore a missing compiler cache #10

The compiler-cache warming task returns only metadata JSON. The compiler files downloaded by mkc are side effects outside redun's output-validity checks.

Relevant code: target/install.py, lines 64–67.

After a successful target installation, clearing ~/.pxt/mkc-cache—or restoring only MKCD2APP_HOME in CI—can leave the installed simulator and redun database intact while removing the compiler. Running target install <version> then reuses the cached metadata and does not restore the compiler. Even target uninstall <version> followed by installation reuses the warming result; only the simulator download is invalidated.

A controlled probe using the real redun scheduler and a fake compiler command confirmed that deleting the simulated compiler cache caused neither reinstall path to rerun mkc.

Expected behavior: installation should validate and repair the compiler artifacts it promises to provision. Otherwise a supposedly installed target can require network access during a later build and fail offline.

This also affects the roadmap's cache redirection: moving the cache under mkcd2app's directories will not fix the missing dependency/output tracking by itself.

Found while reviewing PR #7, toolchain at ce12c48.