Let the sort knob win over the corner handle it sits on (#228)

The CI run for 30de4d3 failed on e2e 37b-ii: dragging the sort knob of
a nine-slice frame resized the frame instead (h 18, lifted null). The
knob's middle sits exactly 8 px left of the frame, which is also the
edge of the corner handle's 8 px tolerance. Until now only the rounding
of the pointer decided which of the two won. The slimmer header bars of
#245 moved that rounding to the corner handle's side. The bug itself
has been there since #228.

- sortLineAt bounds the knob where it is drawn, from a gap to the reach
  left of the frame, instead of by the handle tolerance.
- TextureCanvas passes that gap in image pixels, scaled by the zoom.
- The corner handle stays reachable at the corner itself and to the
  right of and below it.

Tests:
- ./check.sh: 178 files, 2576 tests, CHECK EXIT 0
- GM_STEPS=1,37: ALL PASSED, 26 checks (37b-ii: lifted 6, h 24)
- New unit test: the knob's middle belongs to the knob even inside
  the corner handle's reach. It fails with the old bound.
- Full e2e run not repeated. 37b-ii was the only failure in CI.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/viewport/sort-by-y.test.ts

@@ -297,11 +297,19 @@ describe('the sort line of a texture frame (#228)', () => {
     const rect = { x: 10, y: 20, w: 30, h: 40 };
     expect(sortLineY(rect, 0)).toBe(60);
     expect(sortLineY(rect, 6)).toBe(54);
-    expect(sortLineAt(rect, 6, { x: 4, y: 54 }, 1, 8)).toBe('knob');
-    expect(sortLineAt(rect, 6, { x: 25, y: 54.5 }, 1, 8)).toBe('line');
-    expect(sortLineAt(rect, 6, { x: 25, y: 57 }, 1, 8)).toBeNull();
-    expect(sortLineAt(rect, 6, { x: 0, y: 54 }, 1, 8)).toBeNull();
-    expect(sortLineAt(rect, 6, { x: 45, y: 54 }, 1, 8)).toBeNull();
+    expect(sortLineAt(rect, 6, { x: 4, y: 54 }, 1, 1, 8)).toBe('knob');
+    expect(sortLineAt(rect, 6, { x: 25, y: 54.5 }, 1, 1, 8)).toBe('line');
+    expect(sortLineAt(rect, 6, { x: 25, y: 57 }, 1, 1, 8)).toBeNull();
+    expect(sortLineAt(rect, 6, { x: 0, y: 54 }, 1, 1, 8)).toBeNull();
+    expect(sortLineAt(rect, 6, { x: 45, y: 54 }, 1, 1, 8)).toBeNull();
+  });
+
+  it('takes the middle of its knob even where the corner handle reaches over it', () => {
+    // The editor's sizes at zoom 1: handles grab 8 px around the corner, the knob is drawn from
+    // 4 to 12 px left of the frame, so its middle sits exactly 8 px out – on the handle's edge.
+    const rect = { x: 100, y: 20, w: 24, h: 24 };
+    expect(sortLineAt(rect, 0, { x: 92, y: 44 }, 8, 4, 14)).toBe('knob');
+    expect(sortLineAt(rect, 0, { x: 97, y: 44 }, 8, 4, 14)).toBeNull();
   });
 });