runtime: fix nil and closed channel select semantics

[cpunion]

Summary

• Make nil channel send/recv park instead of nil-dereferencing in the channel runtime.
• Make direct/select sends to closed channels panic from the shared send paths.
• Add focused test/go channel/select coverage and remove only the now-fixed chan/select3.go GOROOT xfails.

Tests

• Reproduced before fix: go test ./test/goroot -run TestGoRootRunCases/chan/select3.go -count=1 -args -goroot "$(go env GOROOT)" -dirs chan -case '^chan/select3\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 20s -build-timeout 3m failed with llgo nil pointer panic in main$1.
• go test ./test/go -run 'Test(NilChannel|ClosedChannel)' -count=1
• go test ./test/go -count=1
• go run ./cmd/llgo test -count=1 ./test/go/channel_select_semantics_test.go
• go test ./cl -run TestRunFromTestgoSelectAllowsKnownInterleavings -count=1
• go test ./test/goroot -run TestGoRootRunCases/chan/select3.go -count=1 -args -goroot "$(go env GOROOT)" -dirs chan -case '^chan/select3\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 20s -build-timeout 3m
• go test ./test/goroot -run TestGoRootRunCases/chan/select3.go -count=1 -args -goroot "$(go env GOROOT)" -dirs chan -case '^chan/select3\.go$' -run-timeout 20s -build-timeout 3m
• Confirmed chan/select2.go still fails with xfail disabled, so its xfail remains.

CI

• Full GOROOT CI is slow/disabled; this PR relies on normal PR CI from the fork branch plus the targeted GOROOT command above.
• I did not push to xgo-dev/llgo. If manual GOROOT workflow_dispatch cannot resolve fork refs, that remains a workflow blocker rather than a reason to push to upstream.

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[cpunion]

Chan/select update pushed in 4a5ee3b9b3a7585ec97b2c8eeca43e7bcf7af09e.

Scope kept to chan/select runtime semantics:

• Bound high-frequency send/recv/select stack temporaries with llvm.stacksave/llvm.stackrestore and stack-backed select ChanOp slices.
• Made blocking runtime Select use stack-allocated selectOp storage and a single inline channel select waiter slot before falling back to the slice.
• Removed the remaining darwin/arm64 xfails for chan/doubleselect.go and chan/select2.go; chan/select3.go was already passing in this PR.
• Added test/go coverage for high-frequency select receive handshakes.

Left out of this PR: chancap.go, closedchan.go, range.go, and rangegen.go xfail entries remain untouched because the current entries are broader baseline/version failures or runoutput stress/timeout coverage, not the select stack/handshake issue fixed here.

Local validation:

• go test ./test/goroot -count=1 -run TestGoRootRunCases -timeout 30m -args -goroot "$(go env GOROOT)" -dirs chan -case '^(chan/(doubleselect|select2|select3)\.go)$' -xfail /tmp/llgo-no-xfail.yaml -progress 30s
• go test ./test/go -count=1
• go test ./ssa
• go test ./cl
• git diff --check

CI is queued/running on the new head.