Replace ScrollableSegmentedControl with our own page tab bar

[andiwand]

#113 vendored ScrollableSegmentedControl rather than drop CocoaPods support for it, and said the alternative was reimplementing the page tabs on UICollectionView in its own PR. This is that PR, so the 832 lines of third-party UIKit leave the repository.

PageTabBar

OpenDocumentReader/PageTabBar.swift is ~200 lines against the same UICollectionView the vendored control was built on, and implements only what the document view ever asked for: text tabs, an underline under the selection, horizontal scrolling once they no longer fit, and .valueChanged on a tap. The storyboard's custom class and the segmentedControl outlet are renamed accordingly.

Tab widths follow one rule in three cases:

an even share fits every title	every tab gets an even share — the common case, and what it looked like before
they all fit, but not evenly	each tab keeps its own width and shares out the slack, so the row still fills
they no longer fit	each tab keeps its own width and the row scrolls

That is the first behaviour change: the old control gave every tab the width of the longest title, so a single long sheet name pushed the rest off screen — with the eight-sheet document I tested against, barely one and a half tabs were visible.

The second is dark mode. Labels are .label/.secondaryLabel and the bar is secondarySystemBackground, instead of hardcoded darkGray, black and #F9F9F9. Tab cells are accessibility elements now too, with the selected trait, and the row scales with Dynamic Type — it is the same 40 points at the default text size and grows from there.

Adjacent fixes the rewrite made obvious

• documentPagesChanged appended segments and activated a fresh height constraint on every call, so a second announcement would have listed every page twice under conflicting constraints. Titles are assigned wholesale and the height constraint is created once.
• The .valueChanged target was added in viewWillAppear, so a second appearance parsed the document twice for a single tap. It moves to viewDidLoad.
• initialSelect is gone. Selecting a tab in code no longer sends .valueChanged — following UISegmentedControl — so there is nothing to suppress.
• doc.pageNames was force-unwrapped.

Verification

• ODR Full/Debug and ODR Lite/Debug Lite build green on the rebased tree, which includes the GoogleMobileAds 13.7 bump from Bump github.com/googleads/swift-package-manager-google-mobile-ads from 12.14.0 to 13.7.0 #120. Clean build, no new warnings.
• 15 new tests, 20 total, all passing. They cover the three width cases, the height metric, the notify-on-tap-only contract, and — by going through Main.storyboard — that the outlet and the custom class actually resolve.
• Driven end to end against a real 8-sheet .ods through Document/odrcore: all eight sheet names reached the bar, and selecting the fourth tab set document.page = 3 and scrolled that tab into view. Rendered the bar in both appearances to check the underline, the even-width case and the scrolling case.

One thing I could not automate on this machine: tapping the tabs by hand in the running app. The simulator is scriptable but osascript has no assistive-access permission here, so the interaction is covered by the tests above driving the real view hierarchy rather than by a literal finger on the screen.

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