Fix undo correctness, crash-recovery, and syntax highlighting bugs from full codebase review

Undo/redo:
- cmd_newline recorded the undo node's position after the cursor moved past
  the split point, so undoing Enter joined the wrong pair of lines.
- Backspace/Delete line-joins recorded UndoType::Newline, whose apply()
  semantics are inverted for a join (forward=split); add UndoType::JoinLines
  with correct forward/backward behavior.
- Regex replace-all, indent/unindent region, kill-to-eol, kill-line,
  kill-region, and delete-word-prev/next mutated the buffer via the
  no-undo-recording raw APIs; they now record undo, grouped atomically via a
  shared UndoGroupGuard.
- Plain-text replace-all with an empty replacement advanced the scan
  position one character too far, skipping adjacent/overlapping matches.

Crash recovery / memory safety:
- Buffer's move ctor/assignment never carried over swap_rec_,
  on_disk_identity_, or the visual-line-mode fields, so the crash-recovery
  journal silently stopped tracking a buffer whenever Editor's
  std::vector<Buffer> reallocated or shifted (e.g. opening/closing files).
  Added SwapManager::Rehome() plus Flush()-before-mutate in
  Editor::AddBuffer/CloseBuffer so the journal's Buffer* key and the
  background writer thread never reference a stale address.
- UndoTree leaked its entire node graph (including all edit text) on every
  buffer close/reload; added ~UndoTree() to free it.
- main.cc didn't restore the terminal if an exception escaped the run loop;
  added an RAII guard around Frontend::Shutdown().

Input handling:
- Terminal frontend used getch() instead of get_wch(), silently dropping
  non-ASCII keyboard input despite linking wide-char ncurses.
- KKeymap's C-k lookup switches on an already-lowercased key, making
  `case 'E'` unreachable dead code; C-k Shift-E silently aliased to C-k e.
- ImGui file picker wasn't modal: keystrokes typed while it was open still
  reached the buffer underneath as edit commands, and Escape didn't close it.
- Gated ImGuiInputHandler's unconditional fprintf/fflush diagnostics behind
  an IMGUI_IH_DEBUG macro, matching QtInputHandler's existing convention.

Syntax highlighting:
- Go/Rust/SQL highlighters weren't stateful, so multi-line /* */ comments
  mis-highlighted as code starting on the second line.
- Python's triple-quoted-string handler didn't re-scan the remainder of a
  line after a string closed mid-line, missing a same-line reopen.
- HighlighterEngine's state_last_contig_ field was declared but unused,
  forcing an O(n) scan of state_cache_ on every stateful lookup; wired it up
  as a real fast path and fixed InvalidateFrom() to keep it in sync.
- kge's :syntax off was silently undone the next frame because
  apply_syntax_to_buffer() re-applied the config default unconditionally
  every frame; added a user-override flag mirroring the existing
  edit-mode-detection guard.

Added regression tests for all of the above (test_undo.cc, test_kkeymap.cc,
test_command_semantics.cc, test_search_replace_flow.cc, and a new
test_syntax_highlighting.cc). Full suite (163 tests) passes, including under
AddressSanitizer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

syntax/PythonHighlighter.cc

@@ -55,31 +55,26 @@ PythonHighlighter::HighlightLineStateful(const Buffer &buf, int row, const LineS
 	std::string s = buf.GetLineString(static_cast<std::size_t>(row));
 	int n         = static_cast<int>(s.size());
 
+	int i = 0;
+
 	// Triple-quoted string continuation uses in_raw_string with raw_delim either "'''" or "\"\"\""
 	if (state.in_raw_string && (state.raw_delim == "'''" || state.raw_delim == "\"\"\"")) {
 		auto pos = s.find(state.raw_delim);
 		if (pos == std::string::npos) {
 			push(out, 0, n, TokenKind::String);
 			return state; // still inside
-		} else {
-			int end = static_cast<int>(pos + static_cast<int>(state.raw_delim.size()));
-			push(out, 0, end, TokenKind::String);
-			// remainder processed normally
-			s                   = s.substr(end);
-			n                   = static_cast<int>(s.size());
-			state.in_raw_string = false;
-			state.raw_delim.clear();
-			// Continue parsing remainder as a separate small loop
-			int base = end;
-			// original offset, but we already emitted to 'out' with base=0; following spans should be from 'end'
-			// For simplicity, mark rest as Default
-			if (n > 0)
-				push(out, base, base + n, TokenKind::Default);
-			return state;
 		}
+		int end = static_cast<int>(pos + static_cast<int>(state.raw_delim.size()));
+		push(out, 0, end, TokenKind::String);
+		state.in_raw_string = false;
+		state.raw_delim.clear();
+		// Resume the normal tokenizer at the closing delimiter's end, on the
+		// same (unmodified) `s`/`n`, so anything after it - including a new
+		// triple-quoted string opening on this same line - is re-scanned
+		// instead of being dumped into a single opaque Default span.
+		i = end;
 	}
 
-	int i = 0;
 	// Detect comment start '#', ignoring inside strings
 	while (i < n) {
 		char c = s[i];