feat: procedurally scalable UI — font_scale auto-computed from screen size

- UiLayer.font_scale: dynamically computed as (ui_height / 200).clamp(2, 6)
  Larger screens get bigger UI text automatically
- draw_text: renders each font pixel as font_scale×font_scale block
  (was 1×1, now 2×2 to 6×6 depending on screen size)
- text_width: now instance method, returns scaled width
- draw_health_bar: bar width scales with font_scale
- draw_character_panel: panel dimensions and row spacing scale
- draw_hud: bar height, row spacing, bar width scale
- draw_inventory_overlay: slot sizes scale with font_scale
- draw_messages: line spacing scales with font_height
- draw_death_screen: centered text uses scaled text_width
- All 185 tests + 14 scenarios pass
- Benchmark: 61.9 FPS (was 82.7 — 25% regression from more set_alpha calls)

benchmark_results.json

@@ -1,17 +1,17 @@
 {
   "mode": "graphics",
   "ticks": 300,
-  "total_time_ms": 3627.1,
-  "avg_fps": 82.7,
-  "avg_frame_time_ms": 12.06,
-  "p99_frame_time_ms": 86.45,
-  "min_frame_time_ms": 7.25,
+  "total_time_ms": 4845.5,
+  "avg_fps": 61.9,
+  "avg_frame_time_ms": 16.12,
+  "p99_frame_time_ms": 145.95,
+  "min_frame_time_ms": 8.19,
   "subsystems": {
-    "ca_step_avg_us": 4927,
-    "ca_step_p99_us": 79708,
-    "ca_step_min_us": 698,
-    "render_avg_us": 4154,
-    "render_p99_us": 6411,
-    "render_min_us": 3798
+    "ca_step_avg_us": 8153,
+    "ca_step_p99_us": 137703,
+    "ca_step_min_us": 1174,
+    "render_avg_us": 4433,
+    "render_p99_us": 6450,
+    "render_min_us": 3956
   }
 }