fix(ai): stop a stray selection hijacking a new-design prompt

A section-heavy new-design prompt ('Design a … page. Include a search
section …') trips is_section_add_request, so requests_new_whole_screen
returns false; the selection-modify bias then routed the whole prompt
into run_modify_turn, where M3's flat-JSONL output was renest-dropped
to nothing ('Could not parse design nodes'). Add a section-add-blind
creation-signal veto to both routing gates so a new-design request
reaches the design pipeline regardless of an active selection.

crates/op-host-desktop/src/chat_session_launch.rs

@@ -240,6 +240,13 @@ fn should_launch_direct_modify(state: &EditorState, user_text: &str) -> bool {
     if op_host_services::chat_intent::requests_new_whole_screen(user_text) {
         return false;
     }
+    // Same intent, but section-add-blind: a full new-page spec that mentions
+    // "section" ("Include a search section…") still reads as new, so a stray
+    // selection can't drag it into modify (measured: a travel-app design with
+    // a node selected fell into run_modify_turn → M3 flat-JSONL → empty).
+    if op_host_services::chat_intent::has_new_screen_creation_signal(user_text) {
+        return false;
+    }
     let keyword_intent = op_host_services::chat_intent::classify_by_keywords(user_text);
     let selected_target_instruction = !state.selection.set.is_empty()
         && keyword_intent != op_host_services::chat_intent::DesignIntent::Chat;

crates/op-host-services/src/chat_intent.rs

@@ -180,8 +180,13 @@ pub fn classify_intent_for_standard_route(
     if !state.selection.set.is_empty() && keyword_intent == DesignIntent::Chat {
         return DesignIntent::Chat;
     }
-    let selected_target_instruction =
-        !state.selection.set.is_empty() && keyword_intent != DesignIntent::Chat;
+    // A selection biases toward modify — UNLESS the prompt is itself a
+    // whole-screen design request (a stray selection must not hijack "Design
+    // a … page" into an edit; `requests_new_whole_screen` misses these when
+    // they mention "section", so check the veto-free creation signal too).
+    let selected_target_instruction = !state.selection.set.is_empty()
+        && keyword_intent != DesignIntent::Chat
+        && !has_new_screen_creation_signal(text);
     if keyword_intent == DesignIntent::Modify || selected_target_instruction {
         return DesignIntent::Modify;
     }
@@ -468,6 +473,23 @@ pub fn requests_new_whole_screen(prompt: &str) -> bool {
     english_requests_new_screen(&prompt.to_lowercase())
 }
 
+/// Core "this is a NEW screen/design" signal: a creation verb aimed at a
+/// page/screen noun. Unlike [`requests_new_whole_screen`] it does NOT veto on
+/// section-add phrasing — a full new-page spec that merely mentions the word
+/// "section" ("Include a search section…", "Deals of the Week section") still
+/// reads as new. Used to stop a stray canvas selection from hijacking a
+/// whole-screen design prompt into the modify path (measured: a travel-app
+/// design with a node selected routed to modify → M3 flat-JSONL → empty).
+pub fn has_new_screen_creation_signal(prompt: &str) -> bool {
+    let cjk_page = ["页面", "页", "屏幕", "屏"]
+        .iter()
+        .any(|k| prompt.contains(k));
+    if cjk_page && DRAW_VERB_CJK.iter().any(|v| prompt.contains(v)) {
+        return true;
+    }
+    english_requests_new_screen(&prompt.to_lowercase())
+}
+
 /// English new-screen detection by the DETERMINER nearest a page/screen noun:
 /// an indefinite article ("a / an / another / new") means a NEW page is being
 /// made ("make a login page", "draw a checkout page"); a definite/demonstrative

crates/op-host-services/src/chat_intent_selection_tests.rs

@@ -82,3 +82,24 @@ fn selection_bias_does_not_hijack_whole_new_screen_or_chat() {
         DesignIntent::Chat
     );
 }
+
+#[test]
+fn selection_does_not_hijack_english_section_heavy_new_design() {
+    // Regression: "Design a … page" whose spec mentions "section" three times
+    // trips `is_section_add_request`, so `requests_new_whole_screen` is false;
+    // a stray selection then dragged the whole new-design prompt into modify
+    // (measured: M3 flat-JSONL → "Could not parse design nodes"). The
+    // creation-signal veto keeps it New.
+    let provider = Scripted;
+    let state = state_with_selected_card();
+    let prompt = "Design a travel booking mobile app explore page. Include a search section with \"Where to?\" input, date picker chips, and guest count. \"Deals of the Week\" section with 2 featured deal cards. Recently viewed section with 2 compact cards. Bottom tab bar. Warm, inviting design with orange accents.";
+    assert!(
+        crate::chat_intent::has_new_screen_creation_signal(prompt),
+        "creation signal must fire on a design-a-page prompt"
+    );
+    assert_ne!(
+        classify_intent_for_standard_route(&provider, &state, prompt, None),
+        DesignIntent::Modify,
+        "a section-heavy new-design prompt must not be hijacked to modify by a selection"
+    );
+}