feat(derive): check form parser availability

Introduce FormValue as the parser-availability bound for #[slhx::form] models, blanket it for FromStr types, and add compile-fail coverage for a form field whose domain type lacks a parser.

req: form/004

req: form/006

slhx-derive/tests/compile_fail.rs

@@ -325,6 +325,60 @@ struct Profile {
     );
 }
 
+#[test]
+fn form_struct_requires_field_parser() {
+    // req: form/004 req: form/006 req: test/003
+    let fixture = Fixture::new("slhx-derive-form-struct-parser-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-form-struct-parser-fail"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+path = "src/lib.rs"
+
+[dependencies]
+slhx = {{ path = {:?} }}
+"#,
+            repo_path("slhx")
+        ),
+    );
+    fixture.write(
+        "build.rs",
+        r#"fn main() {
+    let out = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
+    std::fs::write(
+        out.join("slhx.syms"),
+        "slhx-syms-v1\nform\ttemplates/app.heml::profile\tprofile\t1\nform_field\tprofile\temail\ttrue\tfalse\n",
+    )
+    .unwrap();
+}
+"#,
+    );
+    fixture.write(
+        "src/lib.rs",
+        r#"struct Email;
+
+#[slhx::form("profile")]
+struct Profile {
+    email: Email,
+}
+"#,
+    );
+
+    let output = check_fixture(&fixture);
+
+    assert!(!output.status.success(), "fixture unexpectedly compiled");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("Email: FormValue") || stderr.contains("Email: slhx::FormValue"),
+        "missing parser availability diagnostic in stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn form_handle_accepts_checked_form_model() {
     // req: form/001 req: form/004 req: form/006