Parallelize PaperBanana figure generation with initial paper writing

Dev phase: AI concept figures now generate in background while writer
writes the initial draft. Writer doesn't need figures to write text
(uses \includegraphics refs). After writer finishes, we wait for
figures to complete, then compile.

Saves ~20 minutes on projects with 3 concept figures (7min each was
blocking writer start).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JihaoXin

ark/pipeline.py

@@ -160,6 +160,25 @@ def run_paper_iteration(self) -> bool:
         page_images = self._maybe_generate_page_images()
         visual_review_section = ""
         if page_images:
+            # Load figure manifest to tell reviewer which figures are AI-generated
+            figure_types_section = ""
+            try:
+                from ark.figure_manifest import load_manifest, get_protected_files
+                manifest = load_manifest(self.figures_dir)
+                figures = manifest.get("figures", {})
+                ai_figs = [f for f, info in figures.items()
+                           if info.get("source") in ("paperbanana", "nano_banana")]
+                mpl_figs = [f for f, info in figures.items()
+                            if info.get("source") == "matplotlib"]
+                if ai_figs or mpl_figs:
+                    figure_types_section = "\n\nFigure sources (for review guidance):\n"
+                    if ai_figs:
+                        figure_types_section += f"- AI-generated concept figures (do not flag for matplotlib style): {', '.join(ai_figs)}\n"
+                    if mpl_figs:
+                        figure_types_section += f"- Matplotlib data plots (can flag for code fixes): {', '.join(mpl_figs)}\n"
+            except Exception:
+                pass
+
             visual_review_section = f"""
 
 ## Visual Review
@@ -172,7 +191,7 @@ def run_paper_iteration(self) -> bool:
 - Is the layout professional (alignment, spacing, margins)?
 - Is the information density appropriate?
 - Does the overall visual quality meet research publication standards?
-"""
+{figure_types_section}"""
 
         # 2. Reviewer Agent
         step_num += 1
@@ -1109,14 +1128,18 @@ def _run_dev_phase(self):
         self.log_section("✏️ Writing Initial Paper Draft")
         self._send_dev_phase_telegram("writing", 0, 0)
 
-        # Generate figures from results
+        # Generate matplotlib figures from results (fast, do first)
         self.log_step("Generating figures from experiment results...", "progress")
         self.generate_figures()
 
-        # Generate AI concept figures (Nano Banana) if enabled
+        # Start AI concept figure generation in background (slow, ~7min/fig)
+        # Writer can proceed in parallel — it writes text first, figures are \includegraphics refs
+        nano_banana_future = None
         if self.config.get("figure_generation") == "nano_banana":
-            self.log_step("Generating AI concept figures (Nano Banana)...", "progress")
-            self._generate_nano_banana_figures()
+            from concurrent.futures import ThreadPoolExecutor
+            self._nano_banana_executor = ThreadPoolExecutor(max_workers=1)
+            self.log_step("Starting AI concept figure generation (background)...", "progress")
+            nano_banana_future = self._nano_banana_executor.submit(self._generate_nano_banana_figures)
 
         # Writer produces complete initial draft
         paper_requirements = self.load_paper_requirements()
@@ -1182,6 +1205,17 @@ def _run_dev_phase(self):
 
         self.run_agent("writer", prompt, timeout=3600)
 
+        # Wait for background AI figure generation to complete (if running)
+        if nano_banana_future is not None:
+            self.log_step("Waiting for AI concept figures to complete...", "progress")
+            try:
+                nano_banana_future.result(timeout=1200)  # 20 min max
+                self.log_step("AI concept figures ready", "success")
+            except Exception as e:
+                self.log(f"AI concept figure generation failed: {e}", "WARN")
+            finally:
+                self._nano_banana_executor.shutdown(wait=False)
+
         # Compile initial draft
         self.log_step("Compiling initial draft...", "progress")
         if self.compile_latex():