ci: skip cargo check for specs that exceed runner memory

full-spec-compile has been failing with SIGTERM (143) roughly 25 minutes into
a 240-minute budget, on main as well as on branches, since well before the
current fixes. It is not a timeout and not disk.

Measured cause: microsoft-graph generates 2.4M lines (16,153 operations) and
peaks at ~14.3 GB RSS in a single rustc process during cargo check. A hosted
ubuntu-latest runner has 16 GB, so it is killed. CARGO_BUILD_JOBS=1 changes
the peak by 0.1% — the memory is one rustc type-checking one crate, so
parallelism and sharding do not help.

Adds GENERATE_ONLY_SPECS, currently just microsoft-graph. It is still
generated, which is where most generator defects surface; only the compile
step is skipped. SPEC_COMPILE_FORCE_CHECK=1 runs it anyway where there is
headroom.

Generate-only specs are counted separately and never folded into `passed`,
and the success line degrades to "N compiled cleanly; M generated but not
compiled" — a green run must not read as full verification when one spec was
never compiled.

Full corpus: 53 passed, 0 gen-failed, 0 check-failed, 1 generate-only,
1 skipped (gitea, Swagger 2.0).

Refs: openapi-generator-lnj

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: lightsofapollo