patches: fix 2 blockers from Opus review

BLOCKER #1: accessibilityUpdating flag exception safety.
A Lisp signal (longjmp) during postAccessibilityUpdates left
the re-entrance flag permanently YES, suppressing all future
AX notifications → VoiceOver goes silent randomly.
Fix: specpdl unwind protection (record_unwind_protect_ptr)
resets the flag on any longjmp. All 3 exit points use unbind_to.

BLOCKER #2: static struct buffer *lastBuffer dangling pointer.
Raw C pointer to buffer struct has no GC protection. After
kill-buffer, the pointer dangles.
Fix: file-scope Lisp_Object lastChildFrameBuffer with staticpro.
EQ comparison instead of pointer equality.

Also: revert accidental em-dash → triple-dash in title bar (0007),
fix README factual error (BUF_OVERLAY_MODIFF cache key).

patches/README.txt

@@ -92,7 +92,9 @@ ARCHITECTURE
       text range, line/index/range conversions, frame-for-range,
       range-for-position, and insertion-point-line-number.
     - Maintains a text cache (cachedText / visibleRuns) keyed on
-      BUF_MODIFF, BUF_OVERLAY_MODIFF, and BUF_BEGV (narrowing).
+      BUF_MODIFF and BUF_BEGV (narrowing).  BUF_OVERLAY_MODIFF is
+      tracked separately for notification dispatch (patch 0007)
+      but not for cache invalidation.
       The cache is the single source of truth for all
       index-to-charpos and charpos-to-index mappings.
     - Detects buffer edits (modiff change), cursor movement (point