patches: review pass 4 - fix BUF_MODIFF->BUF_CHARS_MODIFF in P0007

P0007 (announce overlay candidates) incorrectly changed ensureTextCache
to use BUF_MODIFF, causing O(buffer-size) AX text rebuilds on every
font-lock pass.  Reverted to BUF_CHARS_MODIFF throughout.

P0008 (child frame) now cleanly adds only new functionality without
re-introducing BUF_OVERLAY_MODIFF or BUF_MODIFF.

All review blockers and major issues addressed:
- P0000: unbind_to on fall-through path
- P0001: block_input in ns_ax_buffer_text
- P0003: block_input in postCompletionAnnouncementForBuffer; [trims release]
- P0004: block_input in ns_ax_scan_interactive_spans; mojibake ---
- P0006: texinfo semicolons -> periods
- P0007: BUF_CHARS_MODIFF throughout ensureTextCache (no oscillation)
- P0008: childFrameLastBuffer=BVAR(b,name); no BUF_OVERLAY_MODIFF in
  ensureTextCache; voiceoverSetPoint init; cachedOverlayModiffForText removed

git am passes all 9 patches on Linux git 2.43.0.

patches/0004-ns-add-interactive-span-elements-for-Tab-navigation.patch

@@ -1,4 +1,4 @@
-From 589f1536255d1ad28552d8047387fbf3e9288235 Mon Sep 17 00:00:00 2001
+From e270501ae776e953e369fcfdaf5ba2237c3a44c3 Mon Sep 17 00:00:00 2001
 From: Martin Sukany <martin@sukany.cz>
 Date: Sat, 28 Feb 2026 12:58:11 +0100
 Subject: [PATCH 4/8] ns: add interactive span elements for Tab navigation