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.

crates/core/src/export.rs

@@ -172,7 +172,7 @@ fn collect_external_calls(ops: &[IrOp], ir_ids: &HashSet<WordId>, host_ids: &mut
 }
 
 /// Serialize export metadata to JSON (hand-rolled, no serde dependency).
-fn serialize_metadata(m: &ExportMetadata) -> String {
+pub fn serialize_metadata(m: &ExportMetadata) -> String {
     let mut s = String::from("{\n");
     let _ = writeln!(s, "  \"version\": {},", m.version);
     match m.entry_table_index {