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/core/tests/comparison.rs

@@ -12,6 +12,7 @@ use std::sync::OnceLock;
 
 use wafer_core::config::WaferConfig;
 use wafer_core::outer::ForthVM;
+use wafer_core::runtime_native::NativeRuntime;
 
 // -----------------------------------------------------------------------
 // Gforth discovery (cached)
@@ -74,7 +75,7 @@ struct EngineResult {
 
 /// Run Forth code through WAFER (in-process via `ForthVM`).
 fn run_wafer(code: &str) -> EngineResult {
-    let mut vm = ForthVM::new().expect("Failed to create ForthVM");
+    let mut vm = ForthVM::<NativeRuntime>::new().expect("Failed to create ForthVM");
     let mut output = String::new();
     for line in code.lines() {
         let trimmed = line.trim();
@@ -99,7 +100,8 @@ fn run_wafer(code: &str) -> EngineResult {
 
 /// Run Forth code through WAFER with all optimizations enabled.
 fn run_wafer_optimized(code: &str) -> EngineResult {
-    let mut vm = ForthVM::new_with_config(WaferConfig::all()).expect("Failed to create ForthVM");
+    let mut vm = ForthVM::<NativeRuntime>::new_with_config(WaferConfig::all())
+        .expect("Failed to create ForthVM");
     let mut output = String::new();
     for line in code.lines() {
         let trimmed = line.trim();
@@ -809,7 +811,7 @@ fn performance_report() {
 
     // Verify correctness first
     for bench in &benchmarks {
-        let mut vm = ForthVM::new().expect("VM creation failed");
+        let mut vm = ForthVM::<NativeRuntime>::new().expect("VM creation failed");
         for line in bench.define.lines() {
             let trimmed = line.trim();
             if !trimmed.is_empty() {