Perf: update the line index in place; renderers read only visible rows

- PieceTable::Insert/Delete adjust a valid line index directly (shift
  later line starts, add or drop starts for inserted or deleted
  newlines) instead of invalidating it, which forced a byte-by-byte
  rescan of the whole buffer on the next line query. On a 38 MB,
  1M-line buffer, insert + line query drops from 14.7 ms to 0.28 ms.
- The terminal and ImGui renderers fetch only the rows they draw
  (Nrows + GetLineString) instead of Buffer::Rows(), which rebuilt a
  string for every line of the file on the first frame after each edit.
  Qt's line-count uses switch to Nrows().

ImGuiRenderer.cc was syntax-checked with g++ and clang++ against the
vendored ImGui headers; the GUI targets could not be built here (no
SDL2 or Qt6).

Audit findings P1, P2.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

PieceTable.h

@@ -175,6 +175,12 @@ private:
 	// Line index support (rebuilt lazily on demand)
 	void InvalidateLineIndex() const;
 
+	// Keep a valid line index in step with an edit instead of rescanning the
+	// whole buffer on the next query. No-ops if the index is already dirty.
+	void lineIndexOnInsert(std::size_t offset, const char *text, std::size_t len) const;
+
+	void lineIndexOnDelete(std::size_t offset, std::size_t len) const;
+
 	void RebuildLineIndex() const;
 
 	// Underlying storages