runtime: start FP walks from a live caller frame

[cpunion]

Fixes #2149.

Problem

A shallow unrecovered panic built with -O0 -ldflags=-w=false can fall back to the raw clite address dump instead of LLGo's Go-style traceback. It was first noticed when a logging dependency graph and panic were combined in one probe, but the minimized trigger does not depend on log.

Without this PR, output can look like:

panic: shallow-panic

[0x... command-line-arguments.panicSite+0x20, SP = ...]
[0x... command-line-arguments.main+..., SP = ...]

Root cause

llgo_framepointer returned the C helper's own __builtin_frame_address(0). The Go walker dereferenced that stack address after the helper returned, so optimized layouts only worked while stale frame bytes happened to remain intact. In the failing O0/DWARF layout, the saved-FP slot was already overwritten and the walk stopped before collecting a frame.

Change

• Read and return the saved caller FP while the C helper frame is still alive.
• Remove the Go walker's old one-frame skip compensation so existing Caller/Callers depths remain unchanged.
• Cover a shallow panic under O0/O2 with DWARF both enabled and disabled.

This is independent of #2115: that issue owns precise PCLN statement sites. It also does not rely on #2143; optimization can hide the stale-frame bug but is not the correctness fix.

Verification

macOS arm64, Go 1.26.5 / LLVM 19:

• go test -vet=off ./... -count=1 -timeout=20m
• cd runtime && go test -vet=off ./... -count=1 -timeout=15m
• caller/logging/panic/introspection/Stack/line-info acceptance tests
• O0/O2 x DWARF on/off shallow-panic matrix

Ubuntu amd64, Go 1.26.5 / LLVM 19.1.7, offline container limited to 2 CPUs, 15 GiB memory, and 512 PIDs:

• O0/O2 x DWARF on/off shallow-panic matrix

[gemini-code-assist]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.