Hand the contract a launcher it can execute

[frangarcj]

The Intel macOS host built its whole SDK, validated it, and then died naming nothing. From vitasdk/autobuilds#36 after #168 landed:

23:29:27  -- Validated SDK at .../build/vitasdk
23:29:28  -- Static target SDK validation passed
23:29:47  Host dependency audit passed
23:29:47  [100%] Built target finalize-sdk
23:29:47  no such file or directory
23:29:47  make[3]: *** [CMakeFiles/check-toolchain-contract] Error 1

VITASDK_HOST_RUNNER carries the launcher that lets an arm64 machine run what it cross-built — arch -x86_64 — and it arrives from the shell as one string. Interpolated into a VERBATIM COMMAND, a string with a space in it is a single argv[0], so what got looked up on disk was a program named arch -x86_64:

$ cmake -E env FOO=1 "arch -x86_64" cmake -E echo ran
no such file or directory
$ cmake -E env FOO=1 arch -x86_64 cmake -E echo ran
arch: posix_spawnp: /opt/homebrew/bin/cmake: Bad CPU type in executable

The second message is this arm64 laptop refusing to run its own arm64 cmake under arch -x86_64, which is the point: the launcher was reached and did its job. On the runner the binary behind it is the x86_64 SDK, and Rosetta runs it.

x86_64-apple-darwin is cross-built, so check-toolchain-contract is the only thing that executes what it produced. Without it the host publishes unverified — the situation db4d1a593 was written to end.

Second defect of the same commit, and it was hidden behind the first. While run[@]: unbound variable was killing the arm64 leg, the Intel job stopped at no build-machine SDK found for x86_64-apple-darwin (needs arm64-apple-darwin) and never got near the contract. Fixing #168 is what let this one surface.

tests/cmake/host-runner-command.cmake runs the shape the target uses — an environment, a two-word launcher, a command behind it — with cmake -E env as the launcher, because that exists wherever the test runs. Against a runner that is not split it fails three ways:

a launcher with an argument stays two arguments: expected 2 argument(s), got 1
a launcher with several arguments keeps them all: expected 3 argument(s), got 1
a launcher with arguments did not reach the command: status 1, output ''

The split lives in cmake/HostRunner.cmake so the test can call the same function the target does, rather than a copy of it. The shell side is unchanged: smoke_test_bootstrap splits the same string on whitespace into an array, which is correct there.

Every suite in the checks job passes locally.

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AI tools were used in preparing this PR (Claude Opus 5, Anthropic).