hitbox-tool: per-frame collision boxes from sprite sheet alpha -> JSON

- parses the spritec _WxH_CxR naming convention incl. auto-detection of
  integer-upscaled sheets (256x64 named 8x8_4x1 -> 64x64 cells)
- tight alpha bbox per frame with threshold/shrink/pad tuning, row-major
  indices, empty-frame flags; boxes relative to frame origin
- show command draws frames + box outline in the terminal for verification
- go tests: scanning, threshold, shrink/pad clamping, name parsing,
  scale inference, JSON roundtrip

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmdG9GqfSWCzts7AkDwRDh

hitbox-tool/examples/walk_8x8_4x1.hitbox.json

@@ -0,0 +1,58 @@
+{
+  "image": "examples/walk_8x8_4x1.png",
+  "cellW": 64,
+  "cellH": 64,
+  "cols": 4,
+  "rows": 1,
+  "alphaThreshold": 1,
+  "frames": [
+    {
+      "index": 0,
+      "col": 0,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    },
+    {
+      "index": 1,
+      "col": 1,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    },
+    {
+      "index": 2,
+      "col": 2,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    },
+    {
+      "index": 3,
+      "col": 3,
+      "row": 0,
+      "empty": false,
+      "box": {
+        "x": 8,
+        "y": 0,
+        "w": 40,
+        "h": 64
+      }
+    }
+  ]
+}