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/HighlighterEngine.cc

@@ -74,7 +74,22 @@ HighlighterEngine::GetLine(const Buffer &buf, int row, std::uint64_t buf_version
 
 	StatefulHighlighter::LineState prev_state;
 	int start_row = -1;
-	if (!state_cache_.empty()) {
+
+	// Fast path: state_last_contig_ tracks the highest row we've cached state
+	// for, per version. If that row is already below our target, it's
+	// necessarily the best anchor the O(n) scan below would have found, so we
+	// can skip the scan entirely. Re-validated against state_cache_ before
+	// use since InvalidateFrom()/SetHighlighter() may have raced/cleared it.
+	auto contig_it = state_last_contig_.find(buf_version);
+	if (contig_it != state_last_contig_.end() && contig_it->second < row) {
+		auto sc_it = state_cache_.find(contig_it->second);
+		if (sc_it != state_cache_.end() && sc_it->second.version == buf_version) {
+			start_row  = contig_it->second;
+			prev_state = sc_it->second.state;
+		}
+	}
+
+	if (start_row < 0 && !state_cache_.empty()) {
 		// linear search over map (unordered), track best candidate
 		int best = -1;
 		for (const auto &kv: state_cache_) {
@@ -109,6 +124,9 @@ HighlighterEngine::GetLine(const Buffer &buf, int row, std::uint64_t buf_version
 		se.state        = next_state;
 		state_cache_[r] = se;
 		cur_state       = next_state;
+		int &contig     = state_last_contig_[buf_version];
+		if (r > contig)
+			contig = r;
 	}
 
 	// Store in cache and return by value
@@ -139,6 +157,14 @@ HighlighterEngine::InvalidateFrom(int row)
 				++it;
 		}
 	}
+	// A version's tracked contiguous-state row is no longer valid once any
+	// row at or above it has been evicted from state_cache_ above.
+	for (auto it = state_last_contig_.begin(); it != state_last_contig_.end();) {
+		if (it->second >= row)
+			it = state_last_contig_.erase(it);
+		else
+			++it;
+	}
 }