feat(richtext): tint template custom emoji to text color in the editor

Introduce RichTextEmojiView (settable dynamicColor) as the emoji-view-provider
return type; the editor syncs dynamicColor to the current text color each
syncEmojiViews pass so single-color template custom emoji mask-tint to match
surrounding text. Chat composer conforms EmojiTextAttachmentView retroactively
in ChatTextInputPanelNode; article editor wraps in RichTextInlineEmojiView.
Updates hosting tests, demo, and editor docs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>