runtime,cl: preserve panic-site frames across defer and recover

[cpunion]

Problem

LLGo implements panic/recover with longjmp. The jump removes panicking frames before deferred functions run, while Go keeps those logical frames visible to runtime.Caller, runtime.Callers/CallersFrames, and debug.Stack, including after recover until the recovering defer returns.

#2028 owns recoverable hardware faults and fault-context unwinding. This PR preserves ordinary panic snapshots and exposes ordinary and fault snapshots through caller APIs.

Implementation

• capture the physical PC chain before longjmp and splice it below live deferred frames;
• keep the fixed, pointer-free panic snapshot in the current g;
• share the same getg path for ordinary panic and hardware-fault snapshots;
• keep a recovered snapshot observable only while its recovering deferred frame is live;
• attribute deferred execution to the function closing brace and emit an explicit PC/line anchor for panic;
• compile C with frame pointers so fault chains can cross C frames;
• keep wasm and baremetal source sets buildable with no-frame-pointer unwind hooks; snapshot splicing remains disabled there;
• allocate tiny finalizer-test objects on a short-lived goroutine so conservative stack roots cannot make the Go 1.24 Linux CI check intermittent.

The diff against main is +524/-197 across 17 files.

Conformance

• Restored the GOROOT cases directly covered by this PR: issue14646, issue5856, issue33724, devirtualization_nil_panics, and issue11656.
• Removed stale expectations for cases fixed by already-merged changes.
• Reclassified issue25897a, issue45045, and issue54343 only on the platform/version combinations where repeated runs show GC or timeout nondeterminism.

Validation

• macOS arm64, Go 1.26.5: real panic traceback, three sequential recovered C faults, and shallow panic execution at O0/O2 with DWARF both enabled and disabled pass.
• Repeated GOROOT expectation checks pass across Go 1.24, 1.25, and 1.26 on Darwin arm64 and Linux amd64 for the changed cases.
• Standard-runtime builds pass for js/wasm and wasip1/wasm; a cortex-m4 baremetal executable also links successfully.
• The CI-equivalent Go 1.24.2 Linux test/go command passes; the previously intermittent tiny-finalizer test also passes 100 consecutive runs.
• Full local coverage passes: 75.1% across instrumented packages and 97.3% for cl; the complete test/go suite passes.
• Linux amd64, Go 1.26.5: c-shared and c-archive both link into and execute the C export demo; explicit -w=false DWARF source lookup succeeds in both modes.
• The Go export demo passes all 27 assertions.
• runtime/internal/runtime compiles under the host Go toolchain.

Limits

• Statement-level fault lines in untracked functions still need the prebuilt PC-line follow-up.
• Linux cannot reliably name non-dynamic C symbols; captured PCs are retained, but a C frame can be attributed to a neighboring symbol.
• Alternate signal stacks remain outside this PR.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cl/compile.go	92.30%	1 Missing ⚠️

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