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.

1 files changed, 4 insertions, 9 deletions

diff --git a/.github/workflows/build_util.yml b/.github/workflows/build_util.yml
index 52999616d..407ed1e71 100644
--- a/.github/workflows/build_util.yml
+++ b/.github/workflows/build_util.yml
@@ -126,16 +126,11 @@ jobs:
           MEMBROWSE_API_KEY: ${{ secrets.MEMBROWSE_API_KEY }}
         run: |
           # if code-changed is false --> there is no elf -> membrowse target upload with --identical flag
-          # $EX_ARGS is passed for the BOARD it picks, not to scope the targets:
-          # --one-first now chooses a board that can build the -e set (tools/build.py),
-          # so omitting it here would configure a DIFFERENT, empty build dir and upload
-          # --identical for a board that was never compiled. The target list is not
-          # scoped by it - `examples-membrowse-upload` is not `all`, so it passes
-          # through as the aggregate, which has no DEPENDS (hw/bsp/family_support.cmake):
-          # it rebuilds nothing and still records every example, --identical for the
-          # ones without an elf.
+          # deliberately unscoped by $EX_ARGS: keeps the size history on a stable board
+          # per family, at the cost of an --identical-only upload where that board is not
+          # the one the Build step picked (test_ci_metrics pins which families those are)
           BUILD_PY_ARGS="-s ${{ inputs.build-system }} ${{ steps.setup-toolchain.outputs.build_option }} ${{ inputs.build-options }}"
-          python tools/build.py $BUILD_PY_ARGS --target examples-membrowse-upload -j 1 ${{ matrix.arg }} $EX_ARGS
+          python tools/build.py $BUILD_PY_ARGS --target examples-membrowse-upload -j 1 ${{ matrix.arg }}
         shell: bash
 
       - name: Upload Artifacts for Metrics