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

Command.cc

@@ -265,7 +265,6 @@ ensure_cursor_visible(const Editor &ed, Buffer &buf)
 	}
 
 	buf.SetOffsets(rowoffs, coloffs);
-	buf.SetRenderX(rx);
 }
 
 
@@ -3051,7 +3050,6 @@ cmd_show_help(CommandContext &ctx)
 		b.SetDirty(false);
 		b.SetCursor(0, 0);
 		b.SetOffsets(0, 0);
-		b.SetRenderX(0);
 	};
 
 	if (help_index != static_cast<std::size_t>(-1)) {