New mode: Shrink-to-fit rectangle

[pgivel]

Summary

Add a new measurement mode that reverses the current container detection: the user draws a rough rectangle, and the tool contracts each edge inward until it encounters a shape boundary.

Use case

"I roughly know where the element is — show me its exact bounds." Useful when the inside-out container detection picks the wrong region, or when you want to measure a specific element that is not fully enclosed.

Interaction

1. User selects shrink-to-fit mode (keyboard shortcut, e.g. 4).
2. Click & drag to draw a rough bounding rectangle (reuses the modeRectDrag interaction).
3. On mouse release, the rectangle contracts to fit the content.
4. The final rectangle + its dimensions are displayed.

Algorithm

For each side of the user-drawn rectangle, scan inward (row-by-row or column-by-column) through the edge map:

• Top edge: scan rows top→down; stop at the first row where edge-pixel density ≥ threshold.
• Bottom edge: scan rows bottom→up; same.
• Left edge: scan columns left→right; same.
• Right edge: scan columns right→left; same.

The density threshold determines what counts as "encountering a shape" — a single edge pixel is noise. Something like ≥ 30% of pixels in that row/column are edge pixels could work. The existing sensitivity slider could control this, or a separate threshold could be exposed.

This is essentially bounding box refinement using the already-computed Canny edge map.

Implementation notes

• Add shrink_rect_to_content(edge_map, rect, threshold) to core.py — a pure function that returns the contracted rect.
• Expose via a new @pyqtSlot on RulerBackend.
• QML: new mode constant, reuse SelectionOutline for the result display.
• Consider an animation showing the contraction (nice-to-have).

Edge cases

• If no edges are found inside the rectangle on a given side, that side stays at its original position.
• Very small rectangles (< 5px on a side) should be rejected or left as-is.