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>
