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

tests/test_search.cc

@@ -1,46 +0,0 @@
-#include "Test.h"
-#include "OptimizedSearch.h"
-#include <string>
-#include <vector>
-
-
-static std::vector<std::size_t>
-ref_find_all(const std::string &text, const std::string &pat)
-{
-	std::vector<std::size_t> res;
-	if (pat.empty())
-		return res;
-	std::size_t from = 0;
-	while (true) {
-		auto p = text.find(pat, from);
-		if (p == std::string::npos)
-			break;
-		res.push_back(p);
-		from = p + pat.size();
-	}
-	return res;
-}
-
-
-TEST(OptimizedSearch_basic_cases)
-{
-	OptimizedSearch os;
-	struct Case {
-		std::string text;
-		std::string pat;
-	} cases[] = {
-		{"", ""},
-		{"", "a"},
-		{"a", ""},
-		{"a", "a"},
-		{"aaaaa", "aa"},
-		{"hello world", "world"},
-		{"abcabcabc", "abc"},
-		{"the quick brown fox", "fox"},
-	};
-	for (auto &c: cases) {
-		auto got = os.find_all(c.text, c.pat, 0);
-		auto ref = ref_find_all(c.text, c.pat);
-		ASSERT_EQ(got, ref);
-	}
-}