Sketch face extraction misses enclosed regions when squares overlap at a corner

[trailcode]

Summary

When two squares overlap at a corner (e.g. a 2×2 square plus a 1×1 square placed with its center on the top-right corner of the first), Sketch_topo::update_faces() should produce three bounded faces: two L-shaped regions plus the small overlap square. Currently it produces only two faces—the overlap region is missing.

Problem

• The face walker marked both directions of each boundary edge as visited after finding a face (seen_edges.insert(a,b) and seen_edges.insert(b,a)).
• In planar graphs, each directed half-edge (a→b vs b→a) borders exactly one face. Marking both endpoint directions prevents seeding walks for faces that are fully enclosed by already-discovered neighbors.
• Reproducible via the GUI square tool: add a 2×2 square (half-side 1), then a 1×1 square centered on the top-right corner, midpoints off.

Expected fix

• Walk the graph one half-edge at a time; key the visited set on (edge pointer, direction) rather than (node_a, node_b) pairs.
• Use standard leftmost-turn rule with CCW (positive area) cycles as real regions; discard the outer CW cycle.
• Add a regression test exercising the GUI finalize_square_ path.

Acceptance criteria

• Overlapping corner squares produce three faces.
• Existing face tests pass (holes, bridges, mid-splitter rectangles, arcs, dangling edges).
• src/doc/sketch.md documents the half-edge extraction approach.

Related

• Prior face-extraction work: 2D sketch face extraction (update_faces_): attempted cleaner rewrite with angular sorting hangs in some sketches; mid-splitters, bridges, and the orientation filter #134, Sketch arc topology: single edge per arc, intersection splitting, tangent face walker #166
• CHANGELOG.md notes on reverted update_faces_ rewrite and orientation sensitivity