feat(f3): viewport fit/centering math (fit-width / fit-page / center)

Extracts the untested ad-hoc arithmetic from pen_editor_screen._centerPage into
pure, shared, tested functions: fitWidthScale (fill viewport width — the
continuous-single default), fitPageScale (letterboxed min-axis fit), and
centerOffset (top-left translation to center scaled content; negative when it
overflows/scrolls). Powers the viewport's initial transform + the reader
fit-width/fit-page actions.

flutter analyze lib/editor clean; 228/228 tests (+5).

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

lib/editor/layout/viewport_fit.dart

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+// lib/editor/layout/viewport_fit.dart
+//
+// Pure fit/centering math for the viewport's initial transform + the reader
+// "fit width / fit page" actions (F3). Today this lives as untested ad-hoc
+// arithmetic in pen_editor_screen._centerPage; extracting it here makes it
+// testable and shared by the new viewport widget.
+//
+// Content coordinates are scale-1 logical px; the returned scale + offset place
+// content inside the viewport. No widgets beyond dart:ui Size/Offset.
+
+import 'dart:ui' show Offset, Size;
+
+/// Scale so content width fills the viewport width (fit-to-width, the
+/// continuous-single default). 0 for non-positive content width.
+double fitWidthScale(Size content, double viewportWidth) {
+  if (content.width <= 0) return 0;
+  return viewportWidth / content.width;
+}
+
+/// Scale so the whole content fits inside the viewport (letterboxed) —
+/// min(widthFit, heightFit). 0 for non-positive content extents.
+double fitPageScale(Size content, Size viewport) {
+  if (content.width <= 0 || content.height <= 0) return 0;
+  final w = viewport.width / content.width;
+  final h = viewport.height / content.height;
+  return w < h ? w : h;
+}
+
+/// Top-left translation that centers [content] scaled by [scale] within
+/// [viewport]. When the scaled content is larger than the viewport on an axis
+/// the offset is negative (content overflows equally on both sides).
+Offset centerOffset(Size content, Size viewport, double scale) {
+  final scaledW = content.width * scale;
+  final scaledH = content.height * scale;
+  return Offset(
+    (viewport.width - scaledW) / 2.0,
+    (viewport.height - scaledH) / 2.0,
+  );
+}