Add editor column guide controls

[yusufm]

Summary

• add View menu controls for showing a column guide and wrapping at it
• support 80, 100, and 120 column guide widths
• persist guide/wrap preferences and draw the guide in the editor

Tests

• xcodebuild test -workspace "MacDown 3000.xcworkspace" -scheme MacDown -only-testing:MacDownTests/MPPreferencesTests -only-testing:MacDownTests/MPEditorViewSubstitutionTests -destination 'platform=macOS'\n\nRelated to Add configurable column width/wrapping option #44

[schuyler]

Thanks for the PR, @yusufm!

The shape here is solid — prefs, a guide drawn in drawViewBackgroundInRect:, optional hard-wrap, and a View › Column Guide submenu. The wrap path is where most of the risk is.

Issues

• Monospace assumption. Both the guide position (columnGuideXPosition) and the wrap width derive from the advance width of "0" × columns. That only aligns for a fixed-pitch font; MacDown lets users pick any editor font, so with a proportional font the guide line and wrap point drift from the real character column.
• Guide line vs. wrap point diverge. columnGuideXPosition includes textContainerOrigin.x (the inset), but the wrap container width in applyColumnGuidePreferences is charWidth*column + 2*padding with no inset term, so when wrapping is on the drawn line and the actual wrap boundary won't coincide.
• Interaction with existing width-limiting. applyColumnGuidePreferences hard-sets widthTracksTextView and containerSize in both branches. MacDown already manages editor width via editorWidthLimited/editorMaximumWidth + adjustEditorInsets; forcing the container to {CGFLOAT_MAX, CGFLOAT_MAX} when wrap is off can stomp that. Worth confirming width-limited + wrap-at-guide compose.
• Horizontal scrolling/clipping. With widthTracksTextView = NO and a fixed container wider than the viewport, the text view/scroll view needs to be horizontally resizable or text will clip. Is that configured?

Suggestions

• The new menu titles (Show Column Guide, Wrap at Column Guide, 80/100/120 Columns) aren't in Localizable.strings.
• Test gap: the tests cover columnGuideXPosition and the clamp, but not the wrap-mode container sizing — the riskiest part is currently unexercised.
• applyColumnGuidePreferences runs twice within one setupEditor: pass (inset branch + end of setup); collapsing to once would avoid the double work.
• Only 80/100/120 are reachable from the menu, but editorColumnGuideWidth accepts any integer — consider whether to constrain it.

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