compiler: add structured LLVM coroutine builder

[cpunion]

Summary

• add a structured switched-resume coroutine builder with initial, ordinary, and final suspend control flow
• inject frame allocation/free hooks while enforcing their append-only CFG contract
• honor target-width frame size/alignment and LLVM 14/18/22 coroutine ABI boundaries
• validate the real CoroSplit shape on LLVM 14, 18, 19, and 21 in focused CI

This slice only builds the selected coroutine primary. It does not add a synchronous twin, dynamic dispatch, scheduler policy, or a fixed runtime frame ABI. The binding-level switched-resume smoke coverage is merged in cpunion/llvm#3.

Testing

• go test ./ssa -count=1
• go test -race ./ssa -run ^'TestCoroBuilder' -count=1\n- go vet -copylocks=false ./ssa\n- go test -tags=llgo ./ssa -run ^'$'\n- focused local runs with LLVM 14, 18, 19, 20, and LLVM 22 forward compatibility\n- actionlint .github/workflows/coroutine.yml\n

[gemini-code-assist]

Code Review

This pull request introduces ssa/coro.go and its corresponding tests in ssa/coro_test.go. It implements CoroBuilder and related configurations to emit target-independent LLVM switched-resume coroutine control flow and frame allocation. I have no feedback to provide as there are no review comments to assess.

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