Reference-line manager, relative time axis, axis-aware ref-line UX

Reference lines:
- Side-panel cluster (ref_line_widget.py): list of lines with Add/Edit/Remove/
  Clear and a properties dialog (value, colour, line style, tag)
- Lines own their state as RefLineProps (held by MainWindow), drawn with a
  triangle drag-handle; dragging writes the value back and refreshes the list
- Replaces the old centre-only add-and-drag-the-whole-line approach

Comparison:
- Time-axis mode selector: Absolute (seconds) vs Relative (% of each track's own
  length), so a long track and a short one line up by song position. Pure view
  transform (plotspec.apply_x_mode), no recompute
- Overlaid different-length tracks now autorange their x union in absolute mode
  instead of clipping to the first track's span

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ref_line_widget.py

@@ -0,0 +1,153 @@
+"""
+Reference-line management: a side-panel list of custom horizontal markers plus a
+properties dialog.
+
+`RefLineControlWidget` is a pure view over a list of `RefLineProps` owned by the
+main window: it renders the list and emits intents (add / edit / remove / clear).
+`RefLineDialog` edits one line's value, colour, line style, and tag.
+"""
+
+import logging
+
+from PyQt5.QtWidgets import (
+  QWidget, QVBoxLayout, QHBoxLayout, QGroupBox, QListWidget, QPushButton,
+  QDialog, QFormLayout, QDoubleSpinBox, QComboBox, QLineEdit, QColorDialog,
+  QDialogButtonBox,
+)
+from PyQt5.QtGui import QColor
+from PyQt5.QtCore import pyqtSignal
+
+from plotspec import RefLineProps
+
+
+_STYLE_CHOICES = [("Solid", "solid"), ("Dashed", "dash"), ("Dotted", "dot")]
+
+
+class RefLineDialog(QDialog):
+  """Edit one reference line's properties. Read the result via `result_props`."""
+
+  def __init__(self, parent, props: RefLineProps):
+    super().__init__(parent)
+    self.setWindowTitle("Reference line")
+    self._color = props.color
+
+    form = QFormLayout(self)
+
+    self.value_spin = QDoubleSpinBox()
+    self.value_spin.setRange(-1e6, 1e6)
+    self.value_spin.setDecimals(2)
+    self.value_spin.setValue(props.value)
+    form.addRow("Value:", self.value_spin)
+
+    self.color_button = QPushButton()
+    self.color_button.clicked.connect(self._pick_color)
+    self._refresh_color_button()
+    form.addRow("Colour:", self.color_button)
+
+    self.style_combo = QComboBox()
+    for label, key in _STYLE_CHOICES:
+      self.style_combo.addItem(label, key)
+    idx = self.style_combo.findData(props.style)
+    if idx >= 0:
+      self.style_combo.setCurrentIndex(idx)
+    form.addRow("Line style:", self.style_combo)
+
+    self.label_edit = QLineEdit(props.label)
+    self.label_edit.setPlaceholderText("(optional tag)")
+    form.addRow("Tag:", self.label_edit)
+
+    buttons = QDialogButtonBox(QDialogButtonBox.Ok | QDialogButtonBox.Cancel)
+    buttons.accepted.connect(self.accept)
+    buttons.rejected.connect(self.reject)
+    form.addRow(buttons)
+
+  def _pick_color(self):
+    chosen = QColorDialog.getColor(QColor(self._color), self, "Reference line colour")
+    if chosen.isValid():
+      self._color = chosen.name()
+      self._refresh_color_button()
+
+  def _refresh_color_button(self):
+    self.color_button.setText(self._color)
+    # Show the colour as the button's background for a quick read.
+    self.color_button.setStyleSheet(f"background-color: {self._color};")
+
+  def result_props(self) -> RefLineProps:
+    return RefLineProps(
+      value=float(self.value_spin.value()),
+      color=self._color,
+      style=self.style_combo.currentData(),
+      label=self.label_edit.text().strip(),
+    )
+
+
+class RefLineControlWidget(QWidget):
+  """List of reference lines with Add / Edit / Remove / Clear controls."""
+
+  addRequested = pyqtSignal()
+  editRequested = pyqtSignal(int)
+  removeRequested = pyqtSignal(int)
+  clearRequested = pyqtSignal()
+
+  def __init__(self, parent=None):
+    super().__init__(parent)
+    self.logger = logging.getLogger(__name__)
+    self.initUI()
+
+  def initUI(self):
+    layout = QVBoxLayout(self)
+    layout.setContentsMargins(5, 5, 5, 5)
+
+    group_box = QGroupBox("Reference lines")
+    group_layout = QVBoxLayout(group_box)
+
+    self.line_list = QListWidget()
+    self.line_list.setMaximumHeight(110)
+    self.line_list.itemDoubleClicked.connect(self._on_double_click)
+    group_layout.addWidget(self.line_list)
+
+    row = QHBoxLayout()
+    self.add_button = QPushButton("Add")
+    self.add_button.clicked.connect(self.addRequested.emit)
+    row.addWidget(self.add_button)
+
+    self.edit_button = QPushButton("Edit…")
+    self.edit_button.clicked.connect(self._emit_edit)
+    row.addWidget(self.edit_button)
+
+    self.remove_button = QPushButton("Remove")
+    self.remove_button.clicked.connect(self._emit_remove)
+    row.addWidget(self.remove_button)
+
+    self.clear_button = QPushButton("Clear")
+    self.clear_button.clicked.connect(self.clearRequested.emit)
+    row.addWidget(self.clear_button)
+
+    group_layout.addLayout(row)
+    layout.addWidget(group_box)
+
+  def set_lines(self, lines: list[RefLineProps]):
+    """Repopulate the list display from the current props (preserving selection)."""
+    current = self.line_list.currentRow()
+    self.line_list.clear()
+    for p in lines:
+      tag = f"  {p.label}" if p.label else ""
+      self.line_list.addItem(f"{p.value:.2f}{tag}")
+    if 0 <= current < self.line_list.count():
+      self.line_list.setCurrentRow(current)
+
+  def _selected_row(self) -> int:
+    return self.line_list.currentRow()
+
+  def _emit_edit(self):
+    row = self._selected_row()
+    if row >= 0:
+      self.editRequested.emit(row)
+
+  def _emit_remove(self):
+    row = self._selected_row()
+    if row >= 0:
+      self.removeRequested.emit(row)
+
+  def _on_double_click(self, _item):
+    self._emit_edit()