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.h

@@ -79,19 +79,6 @@ public:
 	}
 
 
-	// Mode and flags (mirroring legacy fields)
-	void SetMode(int m)
-	{
-		mode_ = m;
-	}
-
-
-	[[nodiscard]] int Mode() const
-	{
-		return mode_;
-	}
-
-
 	void SetKillChain(bool on)
 	{
 		kill_ = on ? 1 : 0;
@@ -104,18 +91,6 @@ public:
 	}
 
 
-	void SetNoKill(bool on)
-	{
-		no_kill_ = on ? 1 : 0;
-	}
-
-
-	[[nodiscard]] bool NoKill() const
-	{
-		return no_kill_ != 0;
-	}
-
-
 	// Kill ring API
 	void KillRingClear()
 	{
@@ -164,18 +139,6 @@ public:
 	}
 
 
-	void SetDirtyEx(int d)
-	{
-		dirtyex_ = d;
-	}
-
-
-	[[nodiscard]] int DirtyEx() const
-	{
-		return dirtyex_;
-	}
-
-
 	void SetUniversalArg(int uarg, int ucount)
 	{
 		uarg_   = uarg;
@@ -189,12 +152,6 @@ public:
 	}
 
 
-	[[nodiscard]] int UCount() const
-	{
-		return ucount_;
-	}
-
-
 	// --- Universal argument control (C-u) ---
 	// Begin or extend a universal argument (like ke's uarg_start)
 	void UArgStart();
@@ -209,19 +166,6 @@ public:
 	// If no universal argument active, returns 1.
 	int UArgGet();
 
-	// Repeatable command flag: input layer can mark the next command as repeatable
-	void SetRepeatable(bool on)
-	{
-		repeatable_ = on;
-	}
-
-
-	[[nodiscard]] bool Repeatable() const
-	{
-		return repeatable_;
-	}
-
-
 	// Status message storage. Rendering is renderer-dependent; the editor
 	// merely stores the current message and its timestamp.
 	void SetStatus(const std::string &message);
@@ -666,14 +610,10 @@ private:
 	};
 
 	std::size_t rows_ = 0, cols_ = 0;
-	int mode_         = 0;
 	int kill_         = 0; // KILL CHAIN
-	int no_kill_      = 0; // don't kill in delete_row
-	int dirtyex_      = 0;
 	std::string msg_;
 	std::time_t msgtm_ = 0;
 	int uarg_          = 0, ucount_ = 0; // C-u support
-	bool repeatable_   = false; // whether the next command is repeatable
 
 	std::vector<Buffer> buffers_;
 	std::vector<Buffer> *shared_buffers_ = nullptr; // if set, use this instead of buffers_