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

QtFrontend.cc

@@ -474,7 +474,7 @@ protected:
 						left += QStringLiteral(" *");
 
 					// total lines suffix " <n>L"
-					unsigned long lcount = static_cast<unsigned long>(buf->Rows().size());
+					unsigned long lcount = static_cast<unsigned long>(buf->Nrows());
 					left                 += QStringLiteral(" ");
 					left                 += QString::number(static_cast<qlonglong>(lcount));
 					left                 += QStringLiteral("L");
@@ -617,7 +617,7 @@ protected:
 				long nr = static_cast<long>(new_rowoffs) + d_rows;
 				if (nr < 0)
 					nr = 0;
-				const auto nrows = static_cast<long>(buf->Rows().size());
+				const auto nrows = static_cast<long>(buf->Nrows());
 				if (nr > std::max(0L, nrows - 1))
 					nr = std::max(0L, nrows - 1);
 				new_rowoffs = static_cast<std::size_t>(nr);