将 Xynova 笔画延长至 50% 黑色覆盖率

沿现有笔画向空白处增长，保留最小线宽和笔画间隙。41 项测试通过，双 UV 矢量图无重叠。

tests/test_stroke_pattern.py

@@ -116,6 +116,8 @@ def test_boolean_is_not_a_number() -> None:
         ("neighbourhood_mm", 0.0, "must be > 0"),
         ("clearance_mm", -0.01, "must be >= 0"),
         ("candidate_count", 0, "must be >= 1"),
+        ("target_coverage", -0.01, "within [0, 1)"),
+        ("target_coverage", 1.0, "within [0, 1)"),
     ],
 )
 def test_range_rules(key: str, value, expected: str) -> None:
@@ -184,5 +186,6 @@ def test_shipped_revisions_declare_the_expected_block(sensor: str) -> None:
     resolved = stroke_pattern.resolve_stroke_settings(block)
     expected = dict(stroke_pattern.STROKE_DEFAULTS)
     if sensor == "finger-xynova":
-        expected.update(width_min_mm=0.20, clearance_mm=0.03, candidate_count=12)
+        expected.update(width_min_mm=0.20, clearance_mm=0.03, candidate_count=12,
+                        target_coverage=0.50)
     assert resolved == expected