Add swap journaling and group undo/redo with extensive tests.

- Introduced SwapManager for sidecar journaling of buffer mutations, with a safe recovery mechanism.
- Added group undo/redo functionality, allowing atomic grouping of related edits.
- Implemented `SwapRecorder` and integrated it as a callback interface for mutations.
- Added unit tests for swap journaling (save/load/replay) and undo grouping.
- Refactored undo to support group tracking and ID management.
- Updated CMake to include the new tests and swap journaling logic.

tests/test_buffer_rows.cc

@@ -0,0 +1,142 @@
+#include "Test.h"
+#include "Buffer.h"
+#include <algorithm>
+#include <limits>
+#include <string>
+#include <vector>
+
+
+static std::vector<std::string>
+split_lines_preserve_trailing_empty(const std::string &s)
+{
+	std::vector<std::string> out;
+	std::size_t start = 0;
+	for (std::size_t i = 0; i <= s.size(); i++) {
+		if (i == s.size() || s[i] == '\n') {
+			out.push_back(s.substr(start, i - start));
+			start = i + 1;
+		}
+	}
+	if (out.empty())
+		out.push_back(std::string());
+	return out;
+}
+
+
+static std::vector<std::size_t>
+line_starts_for(const std::string &s)
+{
+	std::vector<std::size_t> starts;
+	starts.push_back(0);
+	for (std::size_t i = 0; i < s.size(); i++) {
+		if (s[i] == '\n')
+			starts.push_back(i + 1);
+	}
+	return starts;
+}
+
+
+static std::size_t
+ref_linecol_to_offset(const std::string &s, std::size_t row, std::size_t col)
+{
+	auto starts = line_starts_for(s);
+	if (starts.empty())
+		return 0;
+	if (row >= starts.size())
+		return s.size();
+	std::size_t start = starts[row];
+	std::size_t end   = (row + 1 < starts.size()) ? starts[row + 1] : s.size();
+	if (end > start && s[end - 1] == '\n')
+		end -= 1; // clamp before trailing newline
+	return start + std::min(col, end - start);
+}
+
+
+static void
+check_buffer_matches_model(const Buffer &b, const std::string &model)
+{
+	auto expected_lines = split_lines_preserve_trailing_empty(model);
+	const auto &rows    = b.Rows();
+	ASSERT_EQ(rows.size(), expected_lines.size());
+	ASSERT_EQ(b.Nrows(), rows.size());
+
+	auto starts = line_starts_for(model);
+	ASSERT_EQ(starts.size(), expected_lines.size());
+
+	std::string via_views;
+	for (std::size_t i = 0; i < rows.size(); i++) {
+		ASSERT_EQ(std::string(rows[i]), expected_lines[i]);
+		ASSERT_EQ(b.GetLineString(i), expected_lines[i]);
+
+		std::size_t exp_start = starts[i];
+		std::size_t exp_end   = (i + 1 < starts.size()) ? starts[i + 1] : model.size();
+		auto r                = b.GetLineRange(i);
+		ASSERT_EQ(r.first, exp_start);
+		ASSERT_EQ(r.second, exp_end);
+
+		auto v = b.GetLineView(i);
+		ASSERT_EQ(std::string(v), model.substr(exp_start, exp_end - exp_start));
+		via_views.append(v.data(), v.size());
+	}
+	ASSERT_EQ(via_views, model);
+}
+
+
+TEST (Buffer_RowsCache_MultiLineEdits_StayConsistent)
+{
+	Buffer b;
+	std::string model;
+
+	check_buffer_matches_model(b, model);
+
+	// Insert text and newlines in a few different ways.
+	b.insert_text(0, 0, std::string("abc"));
+	model.insert(0, "abc");
+	check_buffer_matches_model(b, model);
+
+	b.split_line(0, 1); // a\nbc
+	model.insert(ref_linecol_to_offset(model, 0, 1), "\n");
+	check_buffer_matches_model(b, model);
+
+	b.insert_text(1, 2, std::string("X")); // a\nbcX
+	model.insert(ref_linecol_to_offset(model, 1, 2), "X");
+	check_buffer_matches_model(b, model);
+
+	b.join_lines(0); // abcX
+	{
+		std::size_t off = ref_linecol_to_offset(model, 0, std::numeric_limits<std::size_t>::max());
+		if (off < model.size() && model[off] == '\n')
+			model.erase(off, 1);
+	}
+	check_buffer_matches_model(b, model);
+
+	// Insert a multi-line segment in one shot.
+	b.insert_text(0, 2, std::string("\n123\nxyz"));
+	model.insert(ref_linecol_to_offset(model, 0, 2), "\n123\nxyz");
+	check_buffer_matches_model(b, model);
+
+	// Delete spanning across a newline.
+	b.delete_text(0, 1, 5);
+	{
+		std::size_t start  = ref_linecol_to_offset(model, 0, 1);
+		std::size_t actual = std::min<std::size_t>(5, model.size() - start);
+		model.erase(start, actual);
+	}
+	check_buffer_matches_model(b, model);
+
+	// Insert/delete whole rows.
+	b.insert_row(1, std::string_view("ROW"));
+	model.insert(ref_linecol_to_offset(model, 1, 0), "ROW\n");
+	check_buffer_matches_model(b, model);
+
+	b.delete_row(1);
+	{
+		auto starts = line_starts_for(model);
+		if (1 < (int) starts.size()) {
+			std::size_t start = starts[1];
+			std::size_t end   = (2 < starts.size()) ? starts[2] : model.size();
+			model.erase(start, end - start);
+		}
+	}
+	check_buffer_matches_model(b, model);
+}