ci_set_matrix: fall open when no selected family builds anywhere

family_list maps a family to the toolchains that build it, and seven hw/bsp
families are in neither: cxd56, efm32, espressif, f1c100s, pic32mz, py32f0,
same7x. Scoping to one of them intersected to nothing, so every toolchain key was
[], every cmake leg skipped on `if: inputs.build-args != '[]'`, code-metrics took
its no-metrics branch, and the PR went green from a build job that ran no
compiler. The only signal was a stderr line nothing greps for.

Not a coverage regression - master gave the same diff no compile coverage either,
since none of the other families compiles same7x's board.h. What is new is that
the gap used to be masked by the full matrix and is now the whole answer, and
that green now means "ran no compiler" rather than "compiled 64 families".

A selection whose families ALL miss is now unusable rather than empty: it prints
UNSCOPED, which build.yml and .circleci/config.yml already grep to drop the build
extras with it, and emits the full matrix. The two neighbouring cases keep their
own answers - an explicit families: [] is still a legitimate nothing-selected, and
a partial miss still scopes to the families that do build, noting the rest.

The contract test pinned an exact count of fall-open markers, which this would
have broken; it now pins the invariant (every message that emits the full matrix
carries the marker) and was checked to still fail when a marker is removed.

Also corrects the drift guard's note about espressif: hil-build-esp builds its
boards by name, but that job is gated on repository_owner, so on a fork an
espressif-only PR builds nowhere.

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