fix(export): single-source freehand recipe screen+export (P0 step 7, R7)

pdf_service._buildFreehandPdfPath hardcoded its OWN StrokeOptions
(thinning:0.7, streamline:0.5) instead of the shared geometry — the R7
hairline-export divergence. The prior pen-feel commit (streamline 0.5→0.32 on
screen) widened the gap: export still rendered at 0.5.

Extract the ONE perfect_freehand recipe into stroke_geometry.freehandOutlinePoints
(owns thinning/smoothing/streamline/simulatePressure). buildStrokeOutline (screen)
and pdf_service (export, via InkStroke→pfPoints) now both call it, so the
StrokeOptions live in exactly one place and screen↔export can't drift again.
Export now matches screen: thinning 0.85 (kDefaultPenThinning), streamline 0.32.

test/export_geometry_test.dart pins it: buildStrokeOutline traces exactly the
shared outline; default thinning == kDefaultPenThinning; thinning is wired;
empty input is safe.

flutter analyze lib/editor clean; 78/78 tests pass (+4).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/services/pdf_service.dart

@@ -8,6 +8,7 @@ import 'package:path_provider/path_provider.dart';
 import 'package:perfect_freehand/perfect_freehand.dart' as pf;
 import 'package:syncfusion_flutter_pdf/pdf.dart';
 
+import '../editor/engine/stroke_geometry.dart' show freehandOutlinePoints;
 import '../models/ink_stroke.dart';
 import '../models/pen_tool.dart';
 
@@ -304,19 +305,15 @@ class PdfService {
         )
         .toList();
 
-    final outline = pf.getStroke(
-      pfPoints,
-      options: pf.StrokeOptions(
-        size: pixelWidth,
-        // Highlighter keeps constant width; pen/marker taper via thinning=0.7.
-        thinning: isHighlighter ? 0.0 : 0.7,
-        smoothing: 0.5,
-        streamline: 0.5,
-        // Real stylus pressure -> don't simulate; no pressure -> let freehand
-        // fake it based on velocity. Highlighter never simulates.
-        simulatePressure: !hasRealPressure && !isHighlighter,
-        isComplete: true,
-      ),
+    // ONE shared recipe with the on-screen painter (R7): export can no longer
+    // drift from screen. Previously this hardcoded thinning:0.7/streamline:0.5,
+    // which diverged from the screen's 0.85/0.32 → hairline export mismatch.
+    final outline = freehandOutlinePoints(
+      pfPoints: pfPoints,
+      size: pixelWidth,
+      isHighlighter: isHighlighter,
+      hasRealPressure: hasRealPressure,
+      isComplete: true,
     );
 
     if (outline.isEmpty) return null;