Rework layout engine around exact-glyph collision, add tests and docs sync

Replace the old bbox/heuristic placement (scale search rounds, large-font
capping, stratified sampling, fill-retry ladders) with an area-model font
sizing pass feeding a C++ exact-glyph collision engine (centroid-biased
spiral + random probing, HD clearance refinement, density/hole
optimization). Simplify the frontend advanced-params panel and JobParams
type to match the surviving config surface, add a layout-constraints test
suite and a repeatable benchmark tool, and bring docs/*.md back in sync
with current code (plus new TESTING.md and DEPLOYMENT.md).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/core/mask.py

@@ -40,15 +40,12 @@ def normalize_mask_for_fill(mask):
 
 
 def calculate_dynamic_dimensions(base_w, base_h, num_words, avg_len=3, mask_stats=None):
-    if not config.AUTO_EXPAND_CANVAS:
-        return base_w, base_h
-
-    effective_fill = config.TARGET_FILL_RATIO if config.TARGET_FILL_RATIO > 0 else max(config.MIN_ACCEPT_FILL_RATIO, 0.82)
+    effective_fill = config.TARGET_FILL_RATIO if config.TARGET_FILL_RATIO > 0 else 0.45
     mask_fill_ratio = 0.5
     if mask_stats is not None:
         mask_fill_ratio = max(0.05, mask_stats["free_ratio"])
 
-    area_per_word = (config.MIN_READABLE_HEIGHT_PX ** 2) * max(1.0, avg_len) * 1.2
+    area_per_word = (config.MIN_READABLE_HEIGHT_PX ** 2) * max(1.0, avg_len) * 1.05
     required_fillable_area = (num_words * area_per_word * max(1, config.N_REPETITIONS)) / max(effective_fill, 0.1)
     required_canvas_area = required_fillable_area / mask_fill_ratio
     current_area = base_w * base_h
@@ -56,8 +53,15 @@ def calculate_dynamic_dimensions(base_w, base_h, num_words, avg_len=3, mask_stat
         scale_factor = (required_canvas_area / current_area) ** 0.5
         new_w = int(base_w * scale_factor)
         new_h = int(base_h * scale_factor)
-        new_w = ((new_w // 100) + 1) * 100
-        new_h = ((new_h // 100) + 1) * 100
+        # Cap HD canvas to keep render/export under the speed budget.
+        # 6000px 边长对激光/打印足够，再大收益很小但 SVG/PNG 成本陡增。
+        max_edge = 6000
+        if max(new_w, new_h) > max_edge:
+            s = max_edge / max(new_w, new_h)
+            new_w = int(new_w * s)
+            new_h = int(new_h * s)
+        new_w = max(100, ((new_w // 100) + 1) * 100)
+        new_h = max(100, ((new_h // 100) + 1) * 100)
         print(f"[Auto-Size] 扩展画布: {base_w}x{base_h} -> {new_w}x{new_h}")
         return new_w, new_h
     return base_w, base_h