test(axum): close page edge-case mutants

Prove partial constructor defaults, missing multipart-boundary rejection, first-root fingerprint replacement, and unchanged non-root HTML through public and nearest-boundary paths.

req: page_swap/001

req: multipart/003

req: abi/005

PLAN.md

@@ -21,7 +21,7 @@
 - [ ] **State:** In progress — the package-native capped xtask entry point is reachable, rejects unknown packages, propagates mutest failure, and mutation-tests `hemx-core`, `hemx-js`, and the full `hemx-test` package cleanly; full package closure remains.
 - **User value:** maintainers can run one bounded repository command and trust that meaningful Rust logic across every mutation-applicable library is either killed or explicitly justified.
 - **Build:** add a capped `hemx-xtask` mutation command that invokes `/opt/repositories/mutest`/`mutest` through package-native test targets rather than the broken workspace-wide example path; enumerate only current mutation-applicable library/proc-macro packages; finish adversarial tests or simplify code until every survivor is classified; keep equivalent, invariant-only, and infrastructure-inapplicable classifications inspectable and minimal; document the exact local release command in the existing readiness surface.
-- **Blocked by:** none; broad survivors currently remain in `hemx-axum`, `hemx-build`, `hemx-derive`, and `hemx-lsp` outside already-clean focused contracts; the current `hemx-axum` frontier now proves page-mode, response constructors, form accessors/rejections, media-type limits, multipart success/error semantics, sync/async registry dispatch, effect/rejection responses, and embedded runtime delivery mutation-clean; page extraction/response and media-type integration paths are now mutation-clean; remaining app/transport integration paths remain.
+- **Blocked by:** none; broad survivors currently remain in `hemx-axum`, `hemx-build`, `hemx-derive`, and `hemx-lsp` outside already-clean focused contracts; the current `hemx-axum` frontier now proves page-mode, response constructors, form accessors/rejections, media-type limits, multipart success/error semantics, sync/async registry dispatch, effect/rejection responses, and embedded runtime delivery mutation-clean; page extraction/response, media-type integration, partial-constructor defaults, missing multipart-boundary rejection, and root-fingerprint injection are now mutation-clean; handler-registration and infallible runtime-header construction remain before the full package gate can pass.
 - **Proof:** the new xtask mutation command exits zero within its documented bound, covers each applicable package, emits no unexplained missed mutant, and a deliberate adjacent mutation makes it fail. `cargo run -p hemx-xtask -- test` remains green. req: test/020 req: test/021
 
 ## 3. Elect and enforce the release license policy

hemx-axum/src/lib.rs

@@ -1609,6 +1609,18 @@ mod tests {
             .expect("root element is rendered");
         assert_eq!(root.value().attr("data-hemx-fp"), Some("1"));
         assert_eq!(root.text().collect::<String>(), "Docs");
+        assert_eq!(html.matches("data-hemx-fp=").count(), 1);
+        assert!(!html.contains("data-hemx-fp=\"99\""));
+    }
+
+    #[test]
+    fn fingerprint_injection_leaves_html_without_a_hemx_root_unchanged() {
+        let html = "<main>Docs</main>".to_owned();
+        assert_eq!(
+            html_with_root_fingerprint(html.clone(), BuildFingerprint(99)),
+            html
+        );
+        // test req: abi/005
     }
 
     #[test]

hemx-axum/tests/response.rs

@@ -144,6 +144,19 @@ async fn interaction_boundary_skips_unnamed_parts_and_rejects_invalid_multipart(
     let _guard = BOUNDARY_TEST_LOCK
         .lock()
         .unwrap_or_else(|error| error.into_inner());
+    let app = Router::new().route("/mutate", post(bounded_mutation));
+    let missing_boundary = app
+        .clone()
+        .oneshot(
+            Request::post("/mutate")
+                .header(header::CONTENT_TYPE, "multipart/form-data")
+                .body(Body::from("invalid"))
+                .unwrap(),
+        )
+        .await
+        .unwrap();
+    assert_eq!(missing_boundary.status(), StatusCode::BAD_REQUEST);
+
     let accepted = concat!(
         "--b\r\n",
         "Content-Disposition: form-data; filename=\"ignored.txt\"\r\n\r\n",
@@ -153,7 +166,6 @@ async fn interaction_boundary_skips_unnamed_parts_and_rejects_invalid_multipart(
         "1\r\n",
         "--b--\r\n"
     );
-    let app = Router::new().route("/mutate", post(bounded_mutation));
     let response = app
         .clone()
         .oneshot(
@@ -369,6 +381,7 @@ fn page_response_constructors_preserve_mode_and_optional_fingerprint() {
     let partial = PageResponse::partial("<main>Partial</main>");
     assert_eq!(partial.mode, PageMode::Partial);
     assert_eq!(partial.html, "<main>Partial</main>");
+    assert_eq!(partial.title, None);
     assert_eq!(partial.fingerprint, None);
 
     let fingerprint = BuildFingerprint(42);