Implement --native flag for standalone executables

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.

Cargo.lock

@@ -1523,6 +1523,7 @@ dependencies = [
  "clap",
  "rustyline",
  "wafer-core",
+ "wasmtime",
 ]
 
 [[package]]