runtime: short-circuit struct equality before string bodies

[cpunion]

Summary

• Add a cheap, panic-order-preserving mismatch pass for runtime struct equality so scalar fields and string lengths can reject unequal values before string body comparison.
• Add test/go coverage for protected-string interface struct comparisons and interface panic ordering.
• Remove fixed fixedbugs/issue8606b.go GOROOT xfail entries.

GOROOT CI note

Full GOROOT runs are slow/disabled for this slice. This PR uses targeted test/goroot runs with the current machine GOROOT (go env GOROOT = /opt/homebrew/Cellar/go@1.24/1.24.11/libexec) and relies on normal PR CI from the fork branch. No branch was pushed to xgo-dev/llgo; if manual GOROOT workflow_dispatch cannot resolve fork refs, that should be documented rather than pushing to xgo-dev.

Test coverage

New stable coverage is in test/go/interface_string_compare_test.go for the fixed string/interface comparison behavior.

Targeted commands

• Reproduced before fix: go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 10m -args -goroot $(go env GOROOT) -dirs fixedbugs -case "^fixedbugs/issue8606b\.go$" -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 30s (failed: llgo signal exit, go exit 0)
• go test ./test/go -run "TestInterfaceStruct(StringCompareShortCircuits|ComparePreservesInterfacePanicOrder)" -count=1
• go run ./cmd/llgo test -run "TestInterfaceStruct(StringCompareShortCircuits|ComparePreservesInterfacePanicOrder)" -count=1 ./test/go
• go test ./ssa -run TestBinOp -count=1
• go test ./cl -run TestDoesNotExist -count=1
• (cd runtime && go test ./internal/runtime -run TestDoesNotExist -count=1)
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 10m -args -goroot $(go env GOROOT) -dirs fixedbugs -case "^fixedbugs/issue8606b\.go$" -xfail /tmp/llgo-empty-xfail.yaml -run-timeout 30s
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 10m -args -goroot $(go env GOROOT) -dirs fixedbugs -case "^fixedbugs/issue8606b\.go$" -run-timeout 30s

Residual risk

Linux GOROOT entries were removed because they are the same runtime equality root cause, but Linux GOROOT was not run locally on this darwin/arm64 host.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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