patches: fix O(position) lag — O(1) fast path in accessibilityIndexForCharpos:

accessibilityIndexForCharpos: walked composed character sequences
from run.ax_start up to the target charpos offset.  For a run
covering an entire ASCII buffer, chars_in = pt - BUF_BEGV, making
each call O(cursor_position).

This method is called from ensureTextCache on EVERY redisplay frame
(as part of the cache validity check), making each frame O(position)
even when the buffer is completely unchanged.  At line 34,000 of a
large file this is ~1,000,000 iterations per frame.

Fix: when ax_length == length for a run (all single-unit characters),
the ax_index is simply ax_start + chars_in.  O(1) instead of O(N).

This is the symmetric counterpart to the charposForAccessibilityIndex:
fast path added in the previous commit.  Both conversion directions
now run in O(1) for pure-ASCII buffers.

patches/0006-doc-add-VoiceOver-accessibility-section-to-macOS-app.patch

@@ -1,4 +1,4 @@
-From 4341052ad931a98b8453368753b596e7743132b8 Mon Sep 17 00:00:00 2001
+From 0accd96937592f4e9bc95356287367f0c56e0f5b Mon Sep 17 00:00:00 2001
 From: Martin Sukany <martin@sukany.cz>
 Date: Sat, 28 Feb 2026 12:58:11 +0100
 Subject: [PATCH 6/8] doc: add VoiceOver accessibility section to macOS