Properties: a header row over the slot table

Names the four columns -- Name, Type, Value, Unit -- in grey at row size over a
hairline rule, so it labels the table without competing with it.

A fifth column of raw flags= letters was tried alongside it and taken out
again. c, a, s and o are the manifest's own shorthand, and a column of them
told the reader nothing the row was not already saying: c is what the white
field means, a is what an "action" value means. What the flags decide is still
decided by them; it is just not spelled out twice.

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

properties.py

@@ -89,6 +89,7 @@ UNIT_SYMBOLS = {
 EDITABLE_FIELD_BG = "#ffffff"
 READONLY_FIELD_BG = "#e8e8dc"
 LABEL_COLOUR = "#000000"
+HEADING_COLOUR = "#6b6b6b"
 READONLY_TEXT = "#444444"
 ROW_BG = "#ffffff"
 HEADER_BG = "#f2f2f2"
@@ -688,12 +689,33 @@ class PropertiesPanel:
         self.frame.bind("<MouseWheel>", self._on_wheel)
         self.frame.bind("<Shift-MouseWheel>", self._on_shift_wheel)
 
-        for grid_row, slot in enumerate(self.slots):
+        self._build_header_row()
+
+        # Slots start below the header and its rule.
+        for grid_row, slot in enumerate(self.slots, start=2):
             if slot["name"] == "meta":
                 self._build_meta_row(grid_row, slot)
             else:
                 self._build_slot_row(grid_row, slot)
 
+    def _build_header_row(self):
+        """Names the columns.
+
+        Deliberately quiet -- the same size as the rows, in grey, over a hairline rule --
+        so it labels the table without competing with it.
+        """
+        heading_font = tkfont.Font(family="Segoe UI", size=8, weight="bold")
+        headings = ((1, "Name"), (2, "Type"), (3, "Value"), (4, "Unit"))
+
+        for column, text in headings:
+            label = self._tinted(tk.Label(self.rows_frame, text=text, anchor="w",
+                                          bg=ROW_BG, fg=HEADING_COLOUR,
+                                          font=heading_font), ROW_BG)
+            label.grid(row=0, column=column, sticky="w", padx=(2, 8), pady=(4, 2))
+
+        rule = self._tinted(tk.Frame(self.rows_frame, height=1, bg="#c8c8c8"), "#c8c8c8")
+        rule.grid(row=1, column=0, columnspan=5, sticky="ew", pady=(0, 2))
+
     def _row_label(self, slot):
         """'14. Proportional Band' — the manifest slot id, so rows match the box's numbering."""
         return f"{slot['slot_id']:02d}. {humanise(slot['name'])}"