test(#883): align GPU capability infra with the CTSolvers contract

[ocots]

Closes #883.

Problem

test/helpers/capabilities.jl implemented the Handbook's capability-gated-test policy, but partially:

• Asymmetric loud guard. On a kkt runner whose driver broke, test/suite/flows/test_gpu_routing.jl failed loudly (if on_gpu_runner(); @test is_cuda_on()), but test/suite/builders/test_options_forwarding.jl — which imported only is_cuda_on — reported Broken, indistinguishable from a laptop.
• on_gpu_runner() matched RUNNER_NAME exactly (in ("kkt", "occidata")). The self-hosted runners register as kkt-runner / occidata-runner, so the check was dead on CI — the identical bug CTSolvers fixed in CTSolvers#223.
• test/suite/solve/test_canonical.jl guarded a whole GPU block behind a bare if is_cuda_on() with only an @info on the else — a silent skip.
• A duplicate is_cuda_on() still sat in test/runtests.jl, and there was no central enforcement equivalent to CTSolvers' test_environment_contract.jl.

Change — full alignment with the CTSolvers pattern (#190 + #223)

File	Change
test/runtests.jl	TestCapabilities is now a Main-bound module of constants: CUDA_FUNCTIONAL, ON_GPU_RUNNER (matches the kkt/occidata substring of RUNNER_NAME), GPU_EXTENSION_ARMED. Duplicate is_cuda_on() removed.
test/helpers/capabilities.jl	deleted — the functions become the constants above
test/suite/solve/test_canonical.jl, test/suite/builders/test_options_forwarding.jl, test/suite/flows/test_gpu_routing.jl	read Main.TestCapabilities.*; every device guard keeps a Test.@test_skip else; the per-site if on_gpu_runner() block is removed (now central)
test/suite/environment/test_environment_contract.jl	new — asserts the extension is armed on every runner, requires the device on a GPU runner (if ON_GPU_RUNNER; @test CUDA_FUNCTIONAL), and greps the suite for the isdefined-against-Main and unbraced-device-guard anti-patterns
CHANGELOG.md	bullet under [2.2.0-beta] → Testing

No package behaviour change — test infrastructure only.

Verification

• suite/environment + suite/flows/test_gpu_routing + suite/builders/test_options_forwarding: 44 pass, 2 broken (the deliberate @test_skip device sites).
• RUNNER_NAME=kkt-runner julia --project -e 'using Pkg; Pkg.test(; test_args=["test/suite/environment"])' → the contract test fails loudly on @test CUDA_FUNCTIONAL (no device on this machine), proving the loud path fires. Unset → green.
• Full suite running; result will be posted here.

Out of scope

• Adding an occidata job to .github/workflows/CI.yml — kkt is still this repo's only GPU job; ON_GPU_RUNNER matching both is future-proofing.
• Restructuring test_canonical.jl's GPU loop into per-leaf @test_skip.

🤖 Generated with Claude Code

[ocots]

Full suite green on this branch: 2246 pass, 2 broken, 0 fail (14m53s).

• suite/environment/test_environment_contract.jl — 3 pass (extension armed, both anti-pattern greps isempty).
• The 2 Broken are the deliberate @test_skip device sites in test_gpu_routing.jl and test_options_forwarding.jl.
• +3 vs main (2243) = the new contract test's assertions.

Loud-path check: RUNNER_NAME=kkt-runner julia --project -e 'using Pkg; Pkg.test(; test_args=["test/suite/environment"])' → 3 passed, 1 failed (the @test CUDA_FUNCTIONAL fails on this device-less machine, as designed). Unset → green.