runtime: attribute panic caller lines

[cpunion]

Summary

• build on the caller-frame metadata path from runtime: track caller frames for LLGo metadata #1884 and update caller line attribution immediately before panic-prone SSA operations
• preserve the caller-frame snapshot captured at panic time so deferred runtime.Caller(2) can report the panic site
• add LLGo-run test/go coverage for explicit panic, nil field panic, bounds panic CallersFrames, and deferred return caller lines
• remove verified darwin/arm64 xfails for bug347.go, bug348.go, issue14646.go, issue29504.go, issue4562.go, and issue5856.go

Scope

• Left issue27201.go xfailed: it depends on runtime.Stack line output, which is covered by the separate lineinfo/runtime.Stack work.
• Left issue33724.go xfailed: it currently fails because the promoted-method nil access does not panic, which is nil/method semantics rather than caller-line attribution.
• This branch is stacked on the already-open caller-frame metadata PR runtime: track caller frames for LLGo metadata #1884; the final commit is the panic-line attribution delta.

Tests

• go test ./test/go -run TestRuntimePanicCallerLineAttribution -count=1 -timeout=10m
• go test ./test/go -run 'TestRuntime(Caller|Callers|PanicCaller)' -count=1 -timeout=10m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout=15m -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs fixedbugs -case '^(fixedbugs/(bug347|bug348|issue14646|issue29504|issue4562|issue5856)\\.go)$' -xfail /dev/null -run-timeout 2m -build-timeout 5m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout=15m -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs fixedbugs -case '^(fixedbugs/(bug347|bug348|issue14646|issue29504|issue4562|issue5856)\\.go)$' -run-timeout 2m -build-timeout 5m
• go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout=15m -args -goroot /opt/homebrew/Cellar/go@1.24/1.24.11/libexec -dirs fixedbugs -case '^(fixedbugs/(bug347|bug348|issue14646|issue27201|issue29504|issue33724|issue4562|issue5856)\\.go)$' -run-timeout 2m -build-timeout 5m
• go test ./ssa -count=1 -timeout=15m
• go test ./cl -count=1 -timeout=15m
• (cd runtime && go test ./... -count=1 -timeout=15m)
• git diff --check

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 59.63855% with 67 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/instr.go	55.00%	39 Missing and 6 partials ⚠️
cl/compile.go	68.75%	16 Missing and 4 partials ⚠️
ssa/package.go	0.00%	2 Missing ⚠️

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