Runtime abstraction + browser REPL

Decouple ForthVM from wasmtime via a Runtime trait so the same outer
interpreter, compiler, and 200+ word definitions work on both native
(wasmtime) and browser (js-sys WebAssembly API) backends.

Runtime trait (runtime.rs):
- HostAccess trait for memory/global ops inside host function closures
- HostFn type: Box<dyn Fn(&mut dyn HostAccess) -> Result<()>>
- Runtime trait: memory, globals, table, instantiate, call, register

NativeRuntime (runtime_native.rs):
- Wraps wasmtime Engine/Store/Memory/Table/Global/Func
- CallerHostAccess bridges HostAccess to wasmtime Caller API
- Feature-gated behind "native" (default)

outer.rs refactor:
- ForthVM<R: Runtime> — generic over execution backend
- All 87 host functions converted from Func::new closures to HostFn
- All memory access via rt.mem_read/write_*, global access via rt.get/set_*
- Zero logic changes — pure API conversion

wafer-core feature gates:
- default = ["native"] includes wasmtime + all native modules
- Without "native": pure Rust only (outer, codegen, optimizer, dictionary)

Browser REPL (crates/web):
- WebRuntime: js-sys WebAssembly.Memory/Table/Global/Module/Instance
- WaferRepl: wasm-bindgen entry point (evaluate, data_stack, reset)
- WebAssembly.Function with Safari fallback (wrapper module)
- Frontend: dark terminal UI, word panel, init code editor, history
- Build: wasm-pack build --target web

All 452 tests pass (431 unit + 1 benchmark + 9 comparison + 11 compliance).

crates/cli/src/main.rs

@@ -7,6 +7,7 @@ use clap::{Parser, Subcommand};
 use wafer_core::export::{ExportConfig, export_module, serialize_metadata};
 use wafer_core::outer::ForthVM;
 use wafer_core::runner::{run_precompiled_bytes, run_wasm_file};
+use wafer_core::runtime_native::NativeRuntime;
 
 /// 8-byte magic trailer identifying a native WAFER executable.
 const NATIVE_MAGIC: &[u8; 8] = b"WAFEREXE";
@@ -136,7 +137,7 @@ fn cmd_build(
 ) -> anyhow::Result<()> {
     let source = std::fs::read_to_string(file)?;
 
-    let mut vm = ForthVM::new()?;
+    let mut vm = ForthVM::<NativeRuntime>::new()?;
     vm.set_recording(true);
     vm.evaluate(&source)?;
 
@@ -261,7 +262,7 @@ fn cmd_run(file: &str) -> anyhow::Result<()> {
 
 /// `wafer` (REPL) or `wafer program.fth` (evaluate and exit)
 fn cmd_eval_or_repl(file: Option<&str>) -> anyhow::Result<()> {
-    let mut vm = ForthVM::new()?;
+    let mut vm = ForthVM::<NativeRuntime>::new()?;
 
     match file {
         Some(file) => {