Round-1 fixes for R4.7: a caret in heading marks splits at the text; a
split closes and reopens bold, italic and strike, refusing inside links,
code, math and HTML; a task's first text takes no block; retype keeps
task boxes and code indent and takes siblings past the list; an alert
in an item keeps the item's opener; paragraph-before moves a holder's
first text below its opener; delete keeps neighbors from joining; a
heading's text keeps closing hashes and block openers escaped; a heading
level is idempotent, as Typora's menu calls it.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
