runtime: tighten recover to direct deferred calls

[cpunion]

This fixes the fixedbugs recover/defer group by making recover succeed only when called directly by the deferred function currently being run.

Changes:

• track panic values as a stack and gate Recover with a deferred-function token
• avoid broad recover-frame instrumentation for defers that cannot directly recover
• preserve named return values across recover with volatile stack slots for recover-capable functions
• add stable coverage under test/go for issue4066, direct deferred func values, and Go 1.26 issue73916/issue73916b semantics
• remove verified xfail entries for fixedbugs/issue4066.go, fixedbugs/issue73916.go, fixedbugs/issue73916b.go

Local verification:

• go test ./test/go -count=1
• go test ./ssa -count=1
• go test ./cl -count=1
• /opt/homebrew/Cellar/go/1.26.0/libexec/bin/go test ./test/goroot -v -run TestGoRootRunCases -count=1 -goroot /opt/homebrew/Cellar/go/1.26.0/libexec -case 'fixedbugs/issue(4066|73916|73916b)\.go' -directive-mode=runlike -xfail \n- go1.24 GOROOT TestGoRootRunCases fixedbugs/issue4066.go with empty xfail\n\nNote: GC/liveness/finalizer/goroutine cases are intentionally untouched.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 93.04813% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ssa/eh.go	84.37%	9 Missing and 1 partial ⚠️
cl/compile.go	95.74%	1 Missing and 1 partial ⚠️
cl/instr.go	97.22%	1 Missing ⚠️

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