plan9asm: add cross-version instruction coverage and WebAssembly lowering

[cpunion]

Summary

• add operand-form and register-pair coverage across supported Plan 9 assembly architectures
• add official Go WebAssembly Plan 9 assembly parsing, signature inference, LLVM lowering, and executable conformance tests
• lower the official Go wasm runtime ABI: generated go_asm.h offsets, linear-memory SP/FP frames, CTXT and register globals, caller PC/resume dispatch, CALLNORESUME, and RETUNWIND
• translate the official internal/bytealg, internal/chacha8rand, internal/runtime/atomic, and runtime wasm assembly without source patches
• validate Go 1.20 through Go 1.27 GOROOT assembly, ARM GOARM=5/6/7, and ARM64 GOARM64=v8.0
• reject closure-context and unsupported operand boundaries explicitly instead of silently miscompiling them

WebAssembly scope

The zero-value/direct mode preserves plan9asm's existing LLVM/C-compatible wasm calling convention. The explicit Go ABI mode models the official GOARCH=wasm assembly contract used by GOOS=js and GOOS=wasip1; Emscripten ABI/profile selection remains owned by LLGo.

The Go ABI mode covers both the Go 1.20 write-barrier/import shape and the Go 1.21+ native helper shape. It emits LLVM modules for all selected official standard-library wasm assembly files, including generated struct offsets such as gobuf_g.

The execution fixtures translate real GOROOT internal/bytealg, internal/runtime/atomic, and runtime assembly, compile the generated LLVM IR with llc, link with wasm-ld, and run the module with Node. Whole-runtime corpus validation proves translation and LLVM object generation; LLGo R1 integration additionally proves these packages compile and run without their former source patches under Emscripten wasm32 and Memory64.

Validation

• go test ./...
• go test -race ./...
• LLGo R1 via a temporary go.work, then again through the pinned pseudo-version: all target profiles build, and llgo test passes under Emscripten wasm32 and Memory64
• Go 1.20 js/wasm official standard-library corpus: 24 asm files, 18 LLVM modules
• Go 1.21–1.27 js/wasm and wasip1/wasm official standard-library corpora: no unsupported forms or parse failures; every generated module accepted by llc
• Go 1.27 Linux/macOS/Windows corpus, including 386, amd64, arm64, ARM GOARM=5/6/7, and ARM64 GOARM64=v8.0

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