PieceTable: newline test from the line index, memchr rebuild; remove dead code

GetLine and LineColToByteOffset fetched each line's last byte (a piece
lookup, an allocation and a cache write) to see whether it was a newline;
the line index already says so: every line but the last ends at one.
LineColToByteOffset runs on every insert. RebuildLineIndex scans with
memchr instead of byte by byte.

Removed, having no callers: OptimizedSearch (built only into its own
test), UndoNodePool.h, SwapManager::RecordSplit/RecordJoin (replay still
reads SPLIT/JOIN), PieceTable::Reserve/Capacity and its capacity
constructor, Buffer's write-only render column, Editor's mode, no-kill,
dirtyex and repeatable flags and the UCount getter, and
RetryPolicy::Conservative.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

Buffer.h

@@ -5,7 +5,7 @@
  * or scratch document and manages:
  *
  * - Content storage: Uses PieceTable for efficient text operations
- * - Cursor state: Current position (curx_, cury_), rendered column (rx_)
+ * - Cursor state: Current position (curx_, cury_)
  * - Viewport: Scroll offsets (rowoffs_, coloffs_) for display
  * - File backing: Optional association with a file on disk
  * - Undo/Redo: Integrated UndoSystem for operation history
@@ -17,7 +17,6 @@
  *
  * 1. Cursor coordinates:
  *    - (curx_, cury_): Logical character position in the document
- *    - rx_: Rendered column accounting for tab expansion
  *
  * 2. File backing:
  *    - Buffers can be file-backed (associated with a path) or scratch (unnamed)
@@ -119,12 +118,6 @@ public:
 	}
 
 
-	[[nodiscard]] std::size_t Rx() const
-	{
-		return rx_;
-	}
-
-
 	[[nodiscard]] std::size_t Nrows() const
 	{
 		return content_LineCount_();
@@ -394,12 +387,6 @@ public:
 	}
 
 
-	void SetRenderX(const std::size_t rx)
-	{
-		rx_ = rx;
-	}
-
-
 	void SetOffsets(const std::size_t row, const std::size_t col)
 	{
 		rowoffs_ = row;
@@ -740,7 +727,6 @@ private:
 
 	// State mirroring original C struct (without undo_tree)
 	std::size_t curx_    = 0, cury_ = 0; // cursor position in characters
-	std::size_t rx_      = 0; // render x (tabs expanded)
 	std::size_t nrows_   = 0; // number of rows
 	std::size_t rowoffs_ = 0, coloffs_ = 0; // viewport offsets
 	mutable std::vector<Line> rows_; // materialized cache of rows (without trailing newlines)