fix bounds panic messages

[cpunion]

Summary

• preserve failing bounds operands and signedness in generated bounds checks
• route slice/string bounds failures through runtime boundsError messages with len/cap-specific codes
• add stable test/go coverage for bounds panic text and un-xfail fixed GOROOT issue30116 cases

GOROOT note

Full GOROOT CI is slow/disabled locally, so I ran targeted fixedbugs cases only. Machine GOROOT: /opt/homebrew/Cellar/go@1.24/1.24.11/libexec (go1.24.11 darwin/arm64). Normal PR CI should run from the fork branch.

Test/go coverage

Added test/go/bounds_panic_text_test.go covering runtime.Error panic text for signed/unsigned index checks, slice/string/array slicing, len vs capacity wording, and full slice max bounds.

Targeted commands run

• go test ./test/go -run TestBoundsPanicText -count=1 -v
• go test ./test/go -run 'Test(BoundsPanicText|UnsignedIndexBoundsCheck|FullSliceBoundsPanicText)' -count=1 -v\n- go run ./cmd/llgo test -run TestBoundsPanicText -count=1 ./test/go\n- go run ./cmd/llgo test -run 'Test(BoundsPanicText|UnsignedIndexBoundsCheck|FullSliceBoundsPanicText)' -count=1 ./test/go\n- go test ./ssa -run 'TestFromTestdata/utf8|TestFromTestgo/cursor' -count=1\n- go test ./ssa -run Test -count=1\n- go test ./cl -run 'TestRunAndTestFrom(Testgo|Testrt|Testdata|Testlibc)' -count=1\n- go test ./test/goroot -run TestGoRootRunCases -count=1 -v -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/issue30116(u)?\.go$' -xfail /tmp/llgo-no-xfail.yaml -run-timeout 5m -build-timeout 3m\n- go test ./test/goroot -run TestGoRootRunCases -count=1 -v -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/issue30116(u)?\.go$' -run-timeout 5m -build-timeout 3m\n\n## Local note\nA direct go test ./runtime/internal/runtime -count=1 is not applicable in this module layout; that runtime path is validated through LLGo test/go, SSA/CL checks, and the targeted GOROOT run.

[cpunion]

PR #1901 follow-up for head b072c41.

Root cause: the failing test (ubuntu-latest, 19) job failed in cl at TestRunAndTestFromTestlibgo/mapzero. The runtime/compiler change now reports the precise Go-style bounds panic runtime error: index out of range [0] with length 0, but cl/_testlibgo/mapzero/expect.txt still expected the old shorter runtime error: index out of range.

Fix: updated the mapzero golden output to match the new bounds panic message. The functional bounds-message coverage remains in test/go/bounds_panic_text_test.go.

Local tests run:

• go test ./test/go -run TestBoundsPanicText -count=1
• go test ./cl -run 'TestRunAndTestFromTestlibgo/mapzero' -count=1\n- go test ./test/go -count=1\n- go test ./ssa -run 'Test.*' -count=1\n- go test ./cl -run TestRunAndTestFromTestlibgo -count=1\n\nCI status: new head checks have started and are currently pending/queued; I am monitoring run set for b072c41.

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