[ahuber21]

cmake/validate-dispatch-surface.cmake rejects a malformed declaration at
configure time -- duplicate, zero or non-numeric extents, svs::Dynamic listed
explicitly, a malformed level spec, a level with no translation unit. It touches
no build-system state, so it also runs under cmake -P: fourteen fixtures in
tests/cmake/dispatch-surface/ each state the error they expect, and
.github/scripts/check_dispatch_surface.sh drives the validator over all of them
plus the default declaration. That script is a pre-commit hook and a CI job, and
needs no compiler and no build tree.

The new Dispatch Surface workflow adds the two things the script cannot check:
that a default configure leaves the committed dispatch_surface.h
byte-identical, and that a full build against valid-reduced.cmake -- which
shares no fixed extent with the default nine -- compiles and tests. A build
that quietly fell back to the committed header would fail to compile there
rather than pass by accident.

Gotcha: script mode gets no cmake_minimum_required, so the validator sets
CMP0007 and CMP0057 itself. Without them an empty |-field vanishes when the
entry is split, and IN_LIST is not an operator.

Part 3 of 4 of the ISA dispatching v2 milestone.

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