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

ErrorRecovery.h

@@ -52,11 +52,6 @@ struct RetryPolicy {
 	}
 
 
-	// Conservative policy for non-critical operations: fewer attempts, slower retries
-	static RetryPolicy Conservative()
-	{
-		return RetryPolicy{2, std::chrono::milliseconds(200), 2.5, std::chrono::milliseconds(10000)};
-	}
 };
 
 // Retry a function with exponential backoff for transient errors