fix(web): surface Forth messages from host-word throws

A host word signals failure by throwing across the JS boundary, and the
browser runtime reported that exception with its Debug form, so an
empty-stack RESIZE surfaced as

    call_func(134) failed: JsValue(Error: Stack underflow ...)

with the engine's JS stack trace glued on. The thrown message IS the
Forth message, so take it verbatim: 'Stack underflow', the same text the
native CLI prints. Exceptions without a message keep the call context --
those are genuine runtime faults, not Forth throws.

CATCH is unaffected: it reads the throw code from its own channel rather
than parsing messages. Verified against a fresh VM in Node (initSync +
WaferRepl): host-word underflow, compiled-guard underflow, THROW,
unknown word and ' RESIZE CATCH . all match the native CLI.

Ships as v0.2.4.

crates/web/Cargo.toml

@@ -12,7 +12,7 @@ workspace = true
 crate-type = ["cdylib", "rlib"]
 
 [dependencies]
-wafer-core = { path = "../core", version = "0.2.3", default-features = false, features = ["crypto"] }
+wafer-core = { path = "../core", version = "0.2.4", default-features = false, features = ["crypto"] }
 wasm-bindgen = "0.2"
 js-sys = "0.3"
 send_wrapper = { workspace = true }

crates/web/src/runtime_web.rs

@@ -38,6 +38,23 @@ impl WebHostAccess {
     }
 }
 
+/// An exception on its way back out of compiled code. Host words rethrow the
+/// Forth message (`Stack underflow`, an `ABORT"` text, a `THROW` description),
+/// so surface exactly that and nothing else — the JS `Error` carries the whole
+/// engine stack in its message, which is noise to a Forth programmer. Anything
+/// without a message is a genuine runtime fault and keeps the call context.
+fn call_error(fn_index: u32, e: &JsValue) -> anyhow::Error {
+    match Reflect::get(e, &"message".into())
+        .ok()
+        .and_then(|m| m.as_string())
+        .and_then(|m| m.lines().next().map(str::trim).map(str::to_string))
+        .filter(|m| !m.is_empty())
+    {
+        Some(msg) => anyhow::anyhow!("{msg}"),
+        None => anyhow::anyhow!("call_func({fn_index}) failed: {e:?}"),
+    }
+}
+
 impl HostAccess for WebHostAccess {
     fn mem_read_i32(&mut self, addr: u32) -> i32 {
         let view = js_sys::Int32Array::new(&self.buffer());
@@ -134,7 +151,7 @@ impl HostAccess for WebHostAccess {
             .dyn_into()
             .map_err(|_| anyhow::anyhow!("table entry {fn_index} is not a function"))?;
         func.call0(&JsValue::NULL)
-            .map_err(|e| anyhow::anyhow!("call_func({fn_index}) failed: {e:?}"))?;
+            .map_err(|e| call_error(fn_index, &e))?;
         Ok(())
     }
 }
@@ -406,7 +423,7 @@ impl Runtime for WebRuntime {
             .dyn_into()
             .map_err(|_| anyhow::anyhow!("table entry {fn_index} is not callable"))?;
         func.call0(&JsValue::NULL)
-            .map_err(|e| anyhow::anyhow!("call_func({fn_index}) failed: {e:?}"))?;
+            .map_err(|e| call_error(fn_index, &e))?;
         Ok(())
     }