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

PieceTable.h

@@ -52,8 +52,6 @@ class PieceTable {
 public:
 	PieceTable();
 
-	explicit PieceTable(std::size_t initialCapacity);
-
 	// Advanced constructor allowing configuration of consolidation heuristics
 	PieceTable(std::size_t initialCapacity,
 	           std::size_t piece_limit,
@@ -70,9 +68,6 @@ public:
 
 	~PieceTable();
 
-	// Public API mirrors GapBuffer
-	void Reserve(std::size_t newCapacity);
-
 	void AppendChar(char c);
 
 	void Append(const char *s, std::size_t len);
@@ -109,13 +104,6 @@ public:
 	}
 
 
-	[[nodiscard]] std::size_t Capacity() const
-	{
-		// Capacity for piece table isn't directly meaningful; report materialized capacity
-		return materialized_.capacity();
-	}
-
-
 	// ===== New buffer-wide API (Phase 1) =====
 	// Byte-based editing operations
 	void Insert(std::size_t byte_offset, const char *text, std::size_t len);