build_utils: key the caches on the tree, not just the arguments

The eight lru_cache layers take repo-RELATIVE paths - 'hw/bsp/<fam>',
'examples/<ex>/skip.txt', the literal 'hw/bsp' glob - while ci_select._in_repo()
chdirs around every call so one process can classify more than one tree. With no
cwd in the key the second tree gets the first tree's answers.

Reproduced: skip_example('host/bare_api','metro_m0_express') is False at the repo
root and STILL False after chdir into a tree where that board does not exist; only
cache_clear() gave the right answer. It bites the code-size skill's base-vs-branch
worktree compare, /pre-pr, and the first test that points classify_build at a
fixture tree. Master had no caching here, so the hazard arrived with it.

_cwd_cache puts os.getcwd() in the key. The 199-test suite passed before only
because every test happens to pass the real REPO; the new
TestCachesAreKeyedOnTheTree crosses trees deliberately.

Also adds the drift guard the class rule was missing. Ports, hw/mcu, get_deps
tokens and bsp families each have one; the class rule had only a comment claiming
vendor_host.c was the sole "enabled by no example config" case until its removal -
which src/class/bth falsifies today. TestClassesWithNoEnablingExample pins the set
to {bth}, so a class added before its first example, or an example config flipped
to 0, fails here instead of silently selecting nothing on both axes. Verified it
fires by adding a class dir nothing enables.

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