[Based on #131] debug: add Wasmtime WASI source sessions

[cpunion]

Based on #131. Tracks Task 5 of xgo-dev#2164; it does not close the umbrella proposal.

What this adds

• make llgo debug -target=wasip1 use the existing GOOS=wasip1 GOARCH=wasm build path and retain embedded DWARF;
• validate final Wasm DWARF, the versioned llgo.debugger custom-section record, and imported-memory shape before launch;
• locate and require Wasmtime 44+ with its built-in gdbstub;
• locate and require LLDB 22+ with the WebAssembly process plugin;
• start Wasmtime without consuming its single debugger connection during readiness checks, then connect LLDB with the Wasm process plugin;
• load the common LLGo LLDB adapter when Python scripting is available, including marker discovery from a Wasm custom section;
• explicitly downgrade to raw source debugging when a Wasm-aware LLDB (including wasi-sdk 33) is built without Python;
• add a focused Wasmtime/LLDB acceptance fixture and CI lane.

The runtime environment module is generated from the artifact's actual env.memory limits instead of hardcoding an address or reverse-engineering the function at runtime. Self-contained memories remain accepted.

Verified locally

• source-file/line breakpoint in main.increment;
• parameter value=41 and named local result=42;
• Go source backtrace through main.main;
• clean process exit;
• llvm-dwarfdump --verify reports no errors on the final Wasm;
• the common LLGo adapter recognizes schema/runtime-layout/ABI v1 from the Wasm custom section;
• relevant unit/command tests pass;
• go test -timeout 20m -skip '^TestRunPrintfWithStdioNobuf$' ./internal/build passes.

Remaining runtime capability gate

LLGo's current WASI artifact imports shared linear memory. Wasmtime 47's guest-debug gdbstub omits SharedMemory from its RSP address space, so source breakpoints, Wasm locals, and backtraces work, but globals and runtime-backed LLGo formatters cannot yet read guest memory. This was reduced to an independent WAT reproduction and reported upstream:

bytecodealliance/wasmtime#14062

The session reports debugger capability downgrades explicitly and does not claim runtime presentation when it is unavailable.

[gemini-code-assist]

Caution

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