ci: keep hosted runners available to forks

[cpunion]

Problem

The Qiniu runner migration on main selects repository-specific self-hosted runners unconditionally. Fork repositories without those runners cannot execute these jobs: the runner API for cpunion/llgo returned total_count: 0.

Change

• Keep the existing Qiniu selection when the destination repository is xgo-dev/llgo.
• On forks, use the existing matrix OS or GitHub-hosted Ubuntu 24.04. Windows/macOS selections and the test matrices remain unchanged.
• Add a fast workflow-policy test to Format Check. It reads the real workflows, rejects unguarded/inverted runner selectors, checks usable fork OS fallbacks, and checks that upstream overrides and Windows/macOS coverage are preserved.

This is an independent main CI fix, not a WebAssembly runtime change. It is temporarily carried as an explicit dependency in the development R3/R4 stack so that fork CI can run before upstream integration. The branch is based on LLVM 22 main bf3071fdd.

Verification

• go test ./.github/workflowpolicy -count=1 and race testing passed; go vet passed.
• actionlint -shellcheck= passed for all workflows. Existing ShellCheck warnings in unchanged scripts are outside this patch; Actions expressions and runner syntax were checked without suppressing runner-label diagnostics.
• git diff --check passed.

Development stays on cpunion/llgo. No upstream repository settings or runners were modified.

[cpunion]

The LLVM 22 version is now published as 7835a55c9, based on bf3071fdd. Policy unit/race/vet checks and Actions configuration lint passed locally. Format Check runs the new policy test, and Targets is also retained on this PR.

To avoid repeating the entire native build/release/benchmark matrix during the dependency-stack refresh, those heavy runs are canceled here. The same runner-selection changes are inherited by the full active matrix on #235. This does not mark the canceled checks as passing, and per-layer performance reruns are deferred. The new fork Format job now requests ubuntu-24.04, not an unavailable qiniu runner; its actual CI result is still pending.

[tcka4enckoalexei2014-commits]

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[tcka4enckoalexei2014-commits]

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