iOS: paint the page spacers natively while pageSpacersColor is set

[artemlitch]

Problem

Injected pagination writes spacers into the document to separate pages, but only the webview knows where they are. Nothing native can draw against them, because the only way out of the page is the React Native message bridge, which goes through JS.

Change

A new iOS prop pageSpacersColor (ColorValue) turns the feature on. While it is set, the webview registers a WKScriptMessageHandler named pageSpacers and hosts an overlay view inside the WKWebView's own scroll view. Unsetting the color removes both, the same way pageCurlEnabled creates and destroys the curl handler, so a webview without the prop never exposes the painter to its page. The stored color survives destroyWebView, because Fabric leaves a recycled host's old props in place and a remount with the same color never runs the setter again.

The page posts { spacers: [{ top, height }] } in CSS pixels from the top of the document. The handler validates the message, then paints one full-width CALayer per spacer on the overlay. UIKit moves the overlay with the content on every frame, so there is no delegate callback or per-frame sync that could let the bands drift from the text. Each post replaces the previous set, reusing the existing layers, with implicit animations disabled. A new source clears the overlay. Changing the color recolors the layers in place. A message that arrives after the overlay is gone is dropped; only a bad payload warns.

Two more props trim each band: pageSpacersVerticalStartOffset moves the top edge down and pageSpacersVerticalEndOffset moves the bottom edge up, both taken off the height, so highlight outlines at the edge of a page are not covered. A spacer shorter than the offsets paints nothing. pageSpacersDebugLogging gates the count and paint time log, like pageCurlDebugLogging.

Everything goes through the Fabric codegen spec, the old-arch view manager, the TS types and the committed lib, following the page curl props. Android gets the no-op setters the generated interface requires.

Verified

Opened a book on an iPhone 17 Pro simulator with the content frame side of this change. One post per open, 69 spacers painted in 0.2 ms. With a red fill the bands sit exactly on the page gaps, chunk ends included, and follow the content through page turns. Measured in raw screenshots at 3x: a 3 pt start offset moved a band's top edge down 9 px and left its bottom edge in place, the end offset did the reverse, and changing either prop moved the live layers without a repaint. Switching to continuous scroll removed the handler and the overlay; switching back repainted from the new plan. Reopening a document after a remount, which recycles the host, repainted. Bookwise passes the reader background color and 3 pt on both ends, so in normal use the layers are invisible.

The content frame and Bookwise side live in the rekindled worktree and are not part of this PR.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FVfQ8bBXbRyu9DdJQSmJge