patches: add 0009 line index perf fix, update README.txt

New patch 0009 fixes O(L) line scanning in accessibilityLineForIndex:
and accessibilityRangeForLine: by adding a precomputed lineStartOffsets
array built once per cache rebuild.  Queries go from O(L) linear scan
to O(log L) binary search.

README.txt: updated patch listing, text cache section, known limitations
(O(L) issue now resolved), stress test threshold raised to 50,000 lines.

patches/README.txt

@@ -19,6 +19,7 @@ PATCH SERIES
   0006 doc: add VoiceOver accessibility section to macOS appendix
   0007 ns: announce overlay completion candidates for VoiceOver
   0008 ns: announce child frame completion candidates for VoiceOver
+  0009  Performance: precomputed line index for O(log L) line queries
 
 
 OVERVIEW
@@ -327,8 +328,13 @@ TEXT CACHE AND VISIBLE RUNS
   without bumping either modiff counter.  The cache is also
   invalidated when the window tree is rebuilt.  NS_AX_TEXT_CAP = 100,000
   UTF-16 units (~200 KB) caps total exposure; buffers larger than
-  ~50,000 lines are truncated for accessibility purposes.  VoiceOver
-  performance degrades noticeably beyond this threshold.
+  ~50,000 lines are truncated for accessibility purposes.
+
+  A lineStartOffsets array is built during each cache rebuild,
+  recording the AX string index where each line begins.  This
+  makes accessibilityLineForIndex: and accessibilityRangeForLine:
+  O(log L) via binary search instead of O(L) linear scanning.
+  The index is freed and rebuilt alongside the text cache.
 
 
 COMPLETION ANNOUNCEMENTS
@@ -645,10 +651,10 @@ KNOWN LIMITATIONS
     produce incomplete or garbled accessibility text.
 
   - Line counting (accessibilityInsertionPointLineNumber,
-    accessibilityLineForIndex:) uses O(lines) iteration via
-    lineRangeForRange.  For buffers with tens of thousands of visible
-    lines this is acceptable but not optimal.  A line-number cache
-    keyed on cachedTextModiff could reduce this to O(1).
+    accessibilityLineForIndex:) was O(lines) in patches 1-5.
+    Patch 0009 adds a precomputed lineStartOffsets array built
+    once per cache rebuild; queries are now O(log L) via binary
+    search.
 
   - Buffers larger than NS_AX_TEXT_CAP (100,000 UTF-16 units) are
     truncated.  The truncation is silent; AT tools navigating past the
@@ -746,9 +752,10 @@ TESTING CHECKLIST
         *Completions*, Tab to a candidate, Enter to execute, then
         C-x o to switch windows.  Emacs must not hang.
 
-  Stress test:
-    25. Open a large file (>5000 lines).  Navigate with C-v / M-v.
-        Verify no significant lag in VoiceOver speech response.
+  Stress test (patch 0009 line index):
+    25. Open a large file (>50,000 lines).  Navigate to the end with
+        M-> or C-v repeatedly.  VoiceOver speech should remain fluid
+        at all positions (no progressive slowdown).
     26. Open an org-mode file with many folded sections.  Verify that
         folded (invisible) text is not announced during navigation.