Commands: second-pass fixes from the follow-up review

- Motion and delete commands read rows through RowsView (on-demand,
  version-cached) instead of Buffer::Rows(), which rebuilt every line of
  the file after each edit. On a 500k-line buffer after an edit:
  backspace/delete/motions 80-90 ms -> ~0.1 ms; C-u 3000 C-d 273 s ->
  0.25 s. Whole-buffer search scans keep Rows() (cached between edits).
- While a prompt is open only prompt-aware and view-only commands run.
  Others edited the buffer behind the prompt (bypassing the read-only
  check) or switched/closed buffers under a pending confirmation, which
  then acted on the wrong buffer. The ':' command path now applies the
  read-only check too.
- Pasting into a prompt (ImGui) flattens line breaks to spaces instead
  of emitting Newline, which accepted the prompt and sent the rest of
  the paste to the buffer (or ran replace-all).
- The mark region clamps both ends; a stale mark past the end of the
  buffer is treated as end of buffer. This fixes kill-region deleting
  the wrong text and leaving the cursor past end-of-line.
- Kill-line with a count stops at end of buffer instead of killing
  lines above the cursor (regression from the D3 change).
- Undo groups nest, so SmartNewline in visual-line mode is one step.
- Word motions and word deletion treat non-ASCII bytes as word
  characters and so never split a UTF-8 character.
- The universal-argument count saturates at 1,000,000 instead of
  overflowing int.

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

tests/test_paste_split.cc

@@ -102,3 +102,15 @@ TEST(PasteSplit_MultilineCRLF)
 	ASSERT_TRUE(cmds[3].id == CommandId::Newline);
 	ASSERT_TRUE(cmds[4].arg == "line3");
 }
+
+
+// Pasting into a prompt must not produce Newline (which accepts the prompt):
+// line breaks become spaces in a single InsertText.
+TEST(PasteSplit_ForPrompt_FlattensLineBreaks)
+{
+	const auto cmds = SplitPasteForPrompt("foo\nbar\r\nbaz\rqux");
+	ASSERT_EQ(cmds.size(), (std::size_t) 1);
+	ASSERT_TRUE(cmds[0].id == CommandId::InsertText);
+	ASSERT_EQ(cmds[0].arg, std::string("foo bar baz qux"));
+	ASSERT_EQ(SplitPasteForPrompt("").size(), (std::size_t) 0);
+}