feat(html): the pdf annotator reports its changes and takes a tool press

[andiwand]

Why

Each host adds a script over odr.annotation that it should not need: the Android app (editing-bridge.js, opendocument-app/OpenDocument.droid#662), the iOS app (DocumentViewController.pageMessageBridge and pressMarker), and the website (frame-bridge.js). The three copies do the same two things:

1. They count the pending marks after every pointerup, pointercancel and selectionchange, and wait 120 ms because a mark settles 50 ms after the pointer lifts. The reason is that the annotator has no change callback.
2. They decide what a tool button does. Only the page can see the selection, so this logic had to be in a script in the page.

What

• odr.onAnnotationChange({count}) fires on each change to what is pending: a mark, a finished stroke, an undo, a removal and a clear. It does not fire while a stroke is drawn, or when the count does not change.
• odr.annotation.press(tool, {color, width}):
  • Over a selection (with a tool that is not ink), the tool marks the selection once in the colour given, clears the selection, and no tool stays armed.
  • With no selection, the press arms the tool, or disarms it if it is armed already. null disarms.
  • It returns the tool that stays armed.
• odr.annotation.recolor(tool, style): over a selection it marks it once, the same as press. With no selection it restyles the tool if that tool is armed. It returns the tool that stays armed.

The behaviour is the same as the behaviour of the host copies. One detail is different: a mark made once does not change the colour that is set for the armed tool.

Not in this PR

• The callback wiring stays in each host (odr.onEditChange = …, and now odr.onAnnotationChange = …). The host must hook into the callbacks, because the core cannot know the transport (addJavascriptInterface, WKScriptMessageHandler or postMessage). A single odr.connect(post) could take all the callbacks, but a pdf page does not carry editing.js, so no one script could hold it. A second API for the same callbacks gives a host nothing.
• markOnSelection stays false by default. Every host sets it with one setOptions call.

Test

test/browser/annotation/tests.html: 22 new checks, run in headless Chrome. The result is 63 checks and 0 failures. On main, the result is 41 checks and 0 failures.