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

CMakeLists.txt

@@ -389,7 +389,6 @@ if (BUILD_TESTS)
             tests/test_swap_cleanup2.cc
             tests/test_swap_git_editor.cc
             tests/test_piece_table.cc
-            tests/test_search.cc
             tests/test_search_replace_flow.cc
             tests/test_reflow_paragraph.cc
             tests/test_reflow_indented_bullets.cc
@@ -418,7 +417,6 @@ if (BUILD_TESTS)
             SearchHighlight.cc
             KKeymap.cc
             SwapRecorder.h
-            OptimizedSearch.cc
             UndoNode.cc
             UndoTree.cc
             UndoSystem.cc