ci: an empty selection must build nothing, plus selector follow-ups (#3845)

ci: an empty selection must build nothing, plus selector follow-ups

A PR whose build axis legitimately selected nothing rebuilt everything.
build.yml reads .build.families twice - as a |-joined regex, and implicitly
as "is anything selected" - but tested only -z "$FAMILY_REGEX", which an
empty list and a charset-rejected one both satisfy while meaning opposite
things. ci_set_matrix had already returned the correct all-empty matrix;
the fall-open branch discarded it. #3842 and #3840 each spent 74 cmake legs
on it. Branch on the two cases instead, rename FAM_* to FAMILY_*, and cover
the block with a test that extracts it from build.yml and executes it - it
had no test at all, which is how this shipped through two merges.

Follow-ups to the same machinery: glob.escape the repo root at five sites,
so a checkout path containing [ or * stops failing closed; drop the ci-full
label, read after the matrix was already computed and so never functional;
delete 13 mcu:MKL25ZXX / mcu:SAME5X skip tokens matching no board; carry the
rule table in the module docstring, guarded against drift; and pin six
selection behaviours a mutation pass proved untested.

Cut the selector's cost 1.8x (26.0s -> 14.6s) with 0 divergences over 260
paths, and stop scoping the membrowse upload by the PR example filter.

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