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

Editor.cc

@@ -728,15 +728,11 @@ void
 Editor::Reset()
 {
 	rows_    = cols_ = 0;
-	mode_    = 0;
 	kill_    = 0;
-	no_kill_ = 0;
-	dirtyex_ = 0;
 	msg_.clear();
 	msgtm_                = 0;
 	uarg_                 = 0;
 	ucount_               = 0;
-	repeatable_           = false;
 	quit_requested_       = false;
 	quit_confirm_pending_ = false;
 	// Reset close-confirm/save state