fix(derive): keep missing-generation diagnostics author-facing

Replace OUT_DIR/path-shaped proc-macro errors with generated slhx file/module language while preserving the build.rs fix hint.

req: dx/005

req: component/004

slhx-derive/src/lib.rs

@@ -12,7 +12,7 @@ pub fn handler(_attr: TokenStream, item: TokenStream) -> TokenStream {
     let name = function.sig.ident.to_string();
 
     let Some(syms_path) = syms_path() else {
-        let message = "#[slhx::handler] requires OUT_DIR; run inside a Cargo crate with slhx_build::app() in build.rs";
+        let message = "#[slhx::handler] requires generated slhx files; add slhx_build::app().run()? to build.rs or run inside a Cargo crate";
         return quote!(
             #function
             compile_error!(#message);
@@ -20,10 +20,7 @@ pub fn handler(_attr: TokenStream, item: TokenStream) -> TokenStream {
         .into();
     };
     if !syms_path.exists() {
-        let message = format!(
+        let message = "#[slhx::handler] could not find generated slhx symbols; add slhx_build::app().run()? to build.rs or check template generation";
-            "#[slhx::handler] could not find {}; add slhx_build::app().run()? to build.rs or check template generation",
-            syms_path.display()
-        );
         return quote!(
             #function
             compile_error!(#message);
@@ -89,7 +86,7 @@ pub fn form(attr: TokenStream, item: TokenStream) -> TokenStream {
     let form_name = parse_macro_input!(attr as LitStr).value();
     let form_struct = parse_macro_input!(item as ItemStruct);
     let Some(syms_path) = syms_path() else {
-        let message = "#[slhx::form] requires OUT_DIR; run inside a Cargo crate with slhx_build::app() in build.rs";
+        let message = "#[slhx::form] requires generated slhx files; add slhx_build::app().run()? to build.rs or run inside a Cargo crate";
         return quote!(
             #form_struct
             compile_error!(#message);
@@ -183,7 +180,7 @@ fn surface_include() -> String {
     let Some(path) = generated_path("slhx.generated.rs") else {
         return format!(
             " compile_error!({:?}); ",
-            "#[slhx::surface] requires OUT_DIR; run inside a Cargo crate with slhx_build::app() in build.rs"
+            "#[slhx::surface] requires generated slhx files; add slhx_build::app().run()? to build.rs or run inside a Cargo crate"
         );
     };
 
@@ -193,8 +190,7 @@ fn surface_include() -> String {
         format!(
             " compile_error!({:?}); ",
             format!(
-                "#[slhx::surface] could not find {}; add slhx_build::app().run()? to build.rs or check template generation",
+                "#[slhx::surface] could not find generated slhx module; add slhx_build::app().run()? to build.rs or check template generation"
-                path.display()
             )
         )
     }
@@ -301,10 +297,10 @@ struct GeneratedFormField {
 
 fn form_contract_errors(syms_path: &PathBuf, form_name: &str, form_struct: &ItemStruct) -> Vec<String> {
     if !syms_path.exists() {
-        return vec![format!(
+        return vec![
-            "#[slhx::form] could not find {}; add slhx_build::app().run()? to build.rs or check template generation",
+            "#[slhx::form] could not find generated slhx symbols; add slhx_build::app().run()? to build.rs or check template generation"
-            syms_path.display()
+                .to_owned(),
-        )];
+        ];
     }
     let expected = form_fields(syms_path, form_name);
     if expected.is_empty() {

slhx-derive/tests/compile_fail.rs

@@ -218,6 +218,10 @@ fn create() -> impl slhx::IntoEffect {
         stderr.contains("add slhx_build::app().run()? to build.rs"),
         "missing build.rs hint in stderr:\n{stderr}"
     );
+    assert!(
+        !stderr.contains("OUT_DIR"),
+        "diagnostic should not teach Cargo internals:\n{stderr}"
+    );
 }
 
 #[test]
@@ -810,6 +814,10 @@ pub mod ui {}
         stderr.contains("add slhx_build::app().run()? to build.rs"),
         "missing build.rs hint in stderr:\n{stderr}"
     );
+    assert!(
+        !stderr.contains("OUT_DIR"),
+        "diagnostic should not teach Cargo internals:\n{stderr}"
+    );
 }
 
 #[test]