ci: fix nine ways the selection under-selected or mismatched

Every one of these dropped coverage silently - the worst failure mode here,
because the PR still goes green. Found by review, each reproduced first.

Selection rules:

* class_macros derived the config macro from the class DIRECTORY, so a change to
  src/class/midi/midi2_device.c selected the midi_test examples (which do not
  compile it) and never examples/device/midi2_device (the only one that enables
  CFG_TUD_MIDI2, and the only one that does). The file's own macro is unioned in
  where it differs - union, never replace: over-selecting costs a build,
  under-selecting merges a break.
* the ${FAMILY_MCUS} fallback added for espressif fired on any family whose
  _family_mcus came back empty, and _cmake_sets is if()-blind and keeps the FIRST
  definition - so mcx/frdm_mcxn947 answered MCXA15, a token six examples' skip.txt
  names, dropping 12 firmware images CMake builds. Limited now to families that
  never spell set(FAMILY_MCUS ...) at all.
* lib_examples read only an example's top-level CMakeLists.txt/Makefile;
  host/msc_file_explorer_freertos names lib/embedded-cli in src/CMakeLists.txt and
  survived by luck. The whole example tree is scanned. (SEGGER_RTT and rt-thread
  still resolve to nothing: all three references sit inside a LOGGER=rtt guard no
  CI build sets - the documented ruling, not a miss.)
* get_family_boards applied ci_skip_boards/ci_preferred_boards only under
  GITHUB_ACTIONS/CIRCLECI, so the selector answered differently on a laptop than
  on a runner; _prune_buildable forces CI semantics. Its one-board pick also
  abandoned the whole preferred list when entry one could not build the -e set,
  and asked skip_example without the build's -D tokens.
* _config_enables and lib_examples still read with the locale encoding - under
  LC_ALL=C the selector tracebacked on three tracked tusb_config.h files. The
  whole selector and its suite run clean there now.

Workflows:

* the Membrowse Upload step omitted $EX_ARGS, but --one-first now picks the board
  from the -e set, so it configured a different, empty build dir and uploaded
  --identical for a board never compiled. It takes $EX_ARGS for the BOARD; the
  target stays the aggregate, which has no DEPENDS and still records every example.
* blanking FAM_REGEX reset only build_filtered, leaving the build scoped while
  code-metrics took the UNSCOPED branch and diffed a 1-family run against the full
  averaged baseline. All three drop together now, as CircleCI's fall-open does.
* CircleCI's EX_ARGS had no character screen and is used unquoted, and its
  code-metrics job still exit 1'd on an empty metrics set - which a scoped build
  makes a legitimate outcome.
* a `ci-full` PR label now turns the scoping off for one PR. A selector bug
  under-selects silently, and without a label the only ways back to a full matrix
  are accidental.

Performance, since the selector gates every other job: family.cmake texts are read
once rather than per changed directory (a 6,000-file dep bump re-read 84 files
99,892 times) and _scrape_mcu is cached: 2.2s -> 0.29s there, 0.8s -> 0.33s on a
class diff.

Tests: a drift guard for hw/bsp families absent from ci_set_matrix.family_list
(they select zero legs now, where they used to ride the full matrix); the rule-4
port test asserted a SUBSET, which set() satisfies, so it could not fail on the
empty selection it exists to catch; the GITHUB_ENV guard test counted a SUM of two
guards. Drops metrics.py's --only-examples, which nothing called, and applies the
TOTAL scrub to the by-example branch that skipped it.

2 files changed, 28 insertions, 12 deletions

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 2ee124cb3..39a4e7afd 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -68,9 +68,14 @@ jobs:
         with:
           fetch-depth: 0
 
+      # The `ci-full` PR label turns the scoping off for one PR: no selection file is
+      # written, so both matrices and every rig job fall back to the unscoped behaviour.
+      # An escape hatch is the point - a selector bug under-selects SILENTLY, and without
+      # a label the only routes back to a full matrix are accidental (touch an
+      # unclassified path, or break the selector badly enough that it falls open).
       - name: CI selection (PR only)
         id: hil-select
-        if: github.event_name == 'pull_request'
+        if: github.event_name == 'pull_request' && !contains(github.event.pull_request.labels.*.name, 'ci-full')
         env:
           BASE_REF: ${{ github.base_ref }}
         run: |
@@ -166,8 +171,6 @@ jobs:
             fi
           fi
           [ -z "$MATRIX_JSON" ] && MATRIX_JSON=$(python .github/scripts/ci_set_matrix.py)
-          echo "matrix=$MATRIX_JSON"
-          echo "matrix=$MATRIX_JSON" >> $GITHUB_OUTPUT
 
           # Build-axis extras: the per-family example map rides as a side channel
           # (a value inside matrix entries would break CircleCI's family parameter
@@ -188,12 +191,23 @@ jobs:
               # silently match another family's baseline
               case "$FAM_REGEX" in
                 *[!-A-Za-z0-9_\|]*)
-                  echo "::warning::unexpected characters in the family list - unscoped metrics"
+                  echo "::warning::unexpected characters in the family list - dropping the scoping"
                   FAM_REGEX='' ;;
               esac
-              [ -z "$FAM_REGEX" ] && BUILD_FILTERED='false'
+              if [ -z "$FAM_REGEX" ]; then
+                # all three drop together, as CircleCI's fall-open does. Resetting only
+                # build_filtered leaves the build scoped while code-metrics takes the
+                # UNSCOPED branch, diffing a 1-family run against the full averaged
+                # baseline and publishing that as the PR's code-size impact.
+                BUILD_FILTERED='false'
+                EXAMPLE_MAP='{}'
+                MATRIX_JSON=$(python .github/scripts/ci_set_matrix.py)
+              fi
             fi
           fi
+          # emitted once, after every path that can still change it
+          echo "matrix=$MATRIX_JSON"
+          echo "matrix=$MATRIX_JSON" >> $GITHUB_OUTPUT
           echo "example_map=$EXAMPLE_MAP" >> $GITHUB_OUTPUT
           echo "build_filtered=$BUILD_FILTERED" >> $GITHUB_OUTPUT
           echo "build_families_regex=$FAM_REGEX" >> $GITHUB_OUTPUT
diff --git a/.github/workflows/build_util.yml b/.github/workflows/build_util.yml
index dfbd83ee2..52999616d 100644
--- a/.github/workflows/build_util.yml
+++ b/.github/workflows/build_util.yml
@@ -126,14 +126,16 @@ jobs:
           MEMBROWSE_API_KEY: ${{ secrets.MEMBROWSE_API_KEY }}
         run: |
           # if code-changed is false --> there is no elf -> membrowse target upload with --identical flag
-          # Deliberately NOT scoped by $EX_ARGS: <TARGET>-membrowse-upload has no
-          # DEPENDS (hw/bsp/family_support.cmake), so the aggregate rebuilds nothing -
-          # it just records every example, reporting the ones with an elf and
-          # --identical for the rest. Filtering it here would drop the excluded
-          # examples from the dataset membrowse-comment.yml reports against, instead
-          # of recording them as unchanged.
+          # $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.
           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 }}
+          python tools/build.py $BUILD_PY_ARGS --target examples-membrowse-upload -j 1 ${{ matrix.arg }} $EX_ARGS
         shell: bash
 
       - name: Upload Artifacts for Metrics