Polish multiple ruler integration

[pascalpp]

Implements #235 as part of #228. Adds the New Ruler menu command, updates the help book for the active-ruler multi-ruler model, and verifies the unit target.

[pascalpp]

[codex reviewer] The new New Ruler menu item in Free Ruler/Base.lproj/MainMenu.xib:82 needs XIB localization entries. I checked the localized MainMenu.strings files and there is no rWt-KM-qSf.title, so all non-English builds will show the English New Ruler label. Please add translations for de, es, fi, ja, and zh-hans as part of this phase, or set the title programmatically with NSLocalizedString.

[pascalpp]

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• Polish multiple ruler integration #244 👈 (View in Graphite)
• Persist and restore multiple rulers #243
• Route ruler commands to active ruler #242
• Split ruler defaults from live settings #241
• Make grouped ruler windows primary #240
• Add multi-ruler manager and lifecycle #239
• Define first-class ruler instance state #238
• feature: Support Multiple Rulers #246 : 9 other dependent PRs (#253 , #255 , #256 and 6 others)
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[pascalpp]

[codex reviewer] One reset-defaults path still looks stale after the settings-window refactor. RulerSettingsController.resetToDefault replaces the active ruler settings with RulerSettings(defaults: prefs) and refreshes the controls, but it never updates rulerController?.opacity. GroupedRulerController.updateSettings then calls applyStateToWindow, where the window first receives the new settings and then immediately has alphaValue overwritten from the controller’s existing opacity field. The foreground/background slider handlers avoid this by assigning rulerController?.opacity after applying settings, but reset does not. Repro from the existing test data: active ruler foreground opacity is 63%, defaults are 88%, choose Ruler Settings > Reset to Defaults; the label/state move to 88%, but the visible ruler can remain at 0.63 alpha until another opacity/activation path runs. Please update the live opacity on reset and extend testRulerSettingsControllerResetsRulerToDefaults to assert the grouped window alpha.

[pascalpp]

[codex reviewer] Small UI polish issue from the test build: xcodebuild now emits an ibtool notice for Free Ruler/Base.lproj/RulerSettingsControlsView.xib: RSV-fg-value: View is clipping its content. That label is initialized to 100% in a 37 pt text field, so the max opacity value is already at the edge in the new shared settings controls view. Please widen the foreground/background opacity value fields or otherwise let them size to fit so 100% cannot clip in the ruler settings/preferences UI.