Fix adaptive AA vertical edge detection

The previous implementation incorrectly checked `dx == 0` to detect
vertical edges, but dx is modified by Bresenham reduction (line 144:
dx = dx % dy). This caused diagonal lines with integer slopes to
incorrectly use the vertical optimization, resulting in thin diagonal
strokes disappearing.

Root cause identified by cubic-dev-ai review:
- dx is reduced to Bresenham remainder, losing original slope info
- Integer-slope diagonals (e.g., 45° lines where dx == dy) would
  have dx==0 after modulo reduction
- These were incorrectly routed to _span_fill_vertical()
- Thin diagonal strokes collapsed due to sub-pixel span width

Fix:
- Use aa_quality flag instead of dx for vertical edge detection
- aa_quality is set from ORIGINAL dx value before Bresenham reduction
- aa_quality==0 means truly vertical edge (original dx==0)
- aa_quality==2 means needs full 4x4 AA

Testing:
- Build successful with no warnings
- mado-perf shows diagonal lines using full 4x4 AA (correct)
- Vertical lines still benefit from optimization

Author: jserv

src/poly.c

@@ -383,10 +383,14 @@ static void _twin_edge_fill(twin_pixmap_t *pixmap,
              * Only apply to spans >= 16 pixels to avoid branch overhead.
              * Threshold: 16 pixels * 4 samples/pixel = 64 samples
              *
-             * Check if both edges forming this span are vertical (dx=0).
+             * Check aa_quality (not dx) to detect vertical edges.
+             * By this point, dx has been reduced by Bresenham, so dx==0
+	     * incorrectly matches diagonal lines with integer slopes, causing
+	     * thin diagonal strokes to disappear. The aa_quality flag was set
+	     * using the ORIGINAL dx value before Bresenham reduction.
              */
             twin_sfixed_t span_width = a->x - x0;
-            if (edge_start && edge_start->dx == 0 && a->dx == 0 &&
+            if (edge_start && edge_start->aa_quality == 0 && a->aa_quality == 0 &&
                 span_width >= (16 << TWIN_POLY_FIXED_SHIFT)) {
                 /* Both edges vertical and span is wide enough: use optimized
                  * span fill */