fix unbuffered channel range close race

[cpunion]

Summary

• Fix unbuffered channel receive comma-ok results when a sender closes immediately after completing the rendezvous.
• Add focused test/go coverage for unbuffered channel range and direct comma-ok receive.
• Remove the now-passing range.go GOROOT xfail entries.

Testing

• go test ./test/go -run 'Test(RangeUnbufferedChanReceivesFinalValueBeforeClose|UnbufferedChanReceiveOKAfterSenderCloses)$' -count=1
• go run ./cmd/llgo test -run 'Test(RangeUnbufferedChanReceivesFinalValueBeforeClose|UnbufferedChanReceiveOKAfterSenderCloses)$' -count=1 ./test/go/range_channel_close_test.go
• go test ./test/go -run 'TestRangeAssignmentConversions$' -count=1
• go run ./cmd/llgo test -run 'TestRangeAssignmentConversions$' -count=1 ./test/go/range_assignment_conversion_test.go
• go test ./cl -run 'TestSkipUnusedArrayDeref|TestRangeFuncDeferAnalysisHelpers|TestCompileRangeFuncDeferModule|TestRangeFuncDeferHelperNilAndCachePaths|TestNestedRangeFuncDeferAnalysisCombinations' -count=1
• (cd runtime && go test ./internal/runtime -count=1)
• go test ./test/goroot -count=1 -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs . -case '^range.go$' -xfail /tmp/llgo-no-xfail.yaml
• go test ./test/goroot -count=1 -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs . -case '^range.go$'

Notes

• GOROOT CI is slow/disabled for broad runs; this PR adds stable test/go coverage for the fixed behavior.
• Normal PR CI should run from the cpunion fork branch; no branch was pushed to xgo-dev/llgo.
• Nearby typeparam/chans.go still times out with xfail disabled on this machine, so that xfail remains unchanged.

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