perf(coro): fuse synchronous child await transactions

[cpunion]

Summary

• fuse managed-child completion preparation with bounded inline selection
• keep llvm.coro.resume/done/destroy in generated callers while fusing allocation-elided destroy, parent reactivation, and completion consumption
• retain the fully checked scheduler path for real suspension, depth fallback, dynamic frames, panic/recover, Goexit, cancellation, and retained panic traces
• remove superseded compiler-only runtime hooks and bind the new hook identities into required roots and physical descriptor digests
• refresh LLVM 22 LIT/metadata expectations and document the transaction/performance checkpoint

Validation

• runtime core package passes; fused inline-await subset also passes with -race
• native defer, explicit panic, and production time.Sleep E2E tests pass
• key native/wasm pre- and post-CoroSplit await, recover, interface, method, and keepalive tests pass
• all 12 _testmeta fixtures pass in isolated processes
• actual standard-file, direct-syscall, blocking-pipe/timer-progress, and loopback-TCP smoke runs pass
• go build -p=1 ./cmd/llgo passes

The aggregate metadata command was intentionally stopped when one long-lived LLVM process reached about 3.5 GiB RSS; the same 12 fixtures all passed when process-isolated. No process remained afterward.

Measured checkpoint

Darwin arm64, Go 1.26.5, LLVM 22.1.8, full LTO, same io_workload source and independent caches:

• 5,000 standard-file operations, 9-run retired-instruction median: 1,244,109,625 -> 1,153,042,521 (-7.32%)
• cycles median: 449,759,850 -> 442,010,131 (-1.72%)
• stripped binary: 7,797,952 -> 7,667,072 bytes (-1.68%)
• Mach-O __text: 4,152,648 -> 4,073,188 bytes (-1.91%)

Wall-clock file timing was noisy on the shared host, so no stable wall-time percentage is claimed.

Known independent gap

The _testgo/goexit fixture still stops before LIT checking on the pre-existing unsupported outcome-plain terminal outcome recipe goexit gap. This PR updates its ABI expectation but does not expand outcome-plain semantics.