Add `wafer build --native` to produce self-contained native executables.
The approach appends AOT-precompiled WASM and metadata to a copy of the
wafer binary itself, requiring no Rust toolchain at build time.
On startup, the binary checks for an appended payload (8-byte "WAFEREXE"
magic trailer). If found, it deserializes the precompiled module and runs
it directly, skipping CLI argument parsing entirely.
Uses wasmtime's Engine::precompile_module() for AOT compilation at build
time and Module::deserialize() at runtime — instant startup with no JIT.
Binary layout: [wafer binary][precompiled wasm][metadata json][trailer]
Trailer: payload_len(u64 LE) + metadata_len(u64 LE) + "WAFEREXE"
Also refactored runner.rs: extracted shared run_module() to avoid
duplication between run_wasm_bytes() and run_precompiled_bytes().
Made serialize_metadata() public for CLI use.
ok2
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