Wiresheet: grid-snapped layout, visible wire routing, knob legend

Layout rules for tidy and for dragging:
- Boxes snap to whole meta steps live while dragged, not only on release.
- The left-most column and top row clear the canvas edge by at least 30px,
  and neighbours by at least 30px, each rounded up to the next step.
- Box widths round up to the grid so a right edge lands on a grid line.
- Column gaps widen only where wires need lanes, so most stay at exactly 30px.
- A sheet too wide for meta's 0..255 range is laid out at 2, then 3, then 4
  grid squares per step. The multiplier is derived from the sheet's contents,
  so painting and tidying agree on it without recording anything in the file.
  The grid size in Preferences is untouched.

Wire routing:
- No wire may pass under a component. A link with no clear route is shown as
  knobs on both slots instead of a line hidden behind a box.
- Vertical runs hug a box within 15px, never closer than 10px, and two runs
  sharing a gap stand 10px apart.
- Horizontal runs of different links stay 5px apart. Runs from one output pin
  may share, since a fan-out is one signal.
- Straight routes claim their rows first, as their segments are pinned to
  their slots and cannot move later.

Knobs:
- Wire colour with a small white centre when the far end is on this sheet,
  grey when it is on another, half and half when a slot carries both.
- Selectable and deletable like a wire, and raised above the boxes so a click
  reaches them.
- Selecting a same-sheet pair draws a red dashed line between the two knobs.

Also:
- get_box_bounds returns the rectangle actually drawn rather than canvas.bbox,
  which was inflated by the outline stroke and by selection.
- Preferences offers to round the box width to a multiple of the grid.
- The tidy coordinate-limit warning said to lower the grid size; raising it is
  what helps, since the reach is 255 * grid size.
- Tidy Layout moves from its own menu into Edit.

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

app_state.py

@@ -44,6 +44,9 @@ selected_canvas_tag = None    # Tracks the currently clicked/highlighted canvas
 selected_tree_item_id = None  # Tracks the currently right-clicked Treeview item
 link_mark_element = None      # Component marked as a link endpoint via "Link Mark"
 selected_link_tag = None      # Canvas tag of the currently selected (red-highlighted) link wire
+unroutable_link_count = 0     # Links drawn as knobs because no wire could avoid a component
+meta_scale = DEFAULT_GRID_SIZE  # Pixels per meta coordinate step on the painted sheet
+knob_link_ends = {}           # wire tag -> (x1, y1, x2, y2) for a link shown as two knobs
 canvas_link_map = {}          # Canvas link tag -> ET.Element (<link>)
 xml_root_element = None       # Keep a persistent root reference to redraw tree layouts cleanly
 current_sheet_parent = None   # Element whose children are painted; None means the app root