Fix #593: anchors and handle ends are easier to grab than the segments through them

[bflatastic]

Anchors are easier to grab with Direct Selection and the anchor tools: a press just off an anchor no longer slips onto the curve.

Cause: an anchor was picked only within the selection tolerance (3 px), the same reach as its segments. The anchor square is 5 px wide, so a press 3.5 px from its centre, still visually on the point's edge, grabbed the curve instead.

Fix: ToolContext::point_tol sets how far a press reaches for anchors and direction-handle ends: at least 2 px past the drawn point, or the Tolerance preference if that is larger.

• At the default Size 3 that is 4.5 px. Size 7 (9 px squares) gives 6.5 px.
• Anchors and handles are tested before segments, so when both are within reach the anchor wins. Segments, objects and area-type frame edges keep the plain tolerance.
• Used by:
  • Direct Selection: press, Highlight anchors on mouse over, blend spine points and handles.
  • Anchor Point, Delete Anchor Point (including Alt from Add Anchor Point) and Scissors on an anchor.
  • The Pen's Alt conversions. The Pen's close/continue/auto-delete reach stays 5 px unless the points are drawn larger.
• Of the topmost path with anchors in reach, the nearest anchor is picked (before, the first one listed).
• anchor_size (Selection & Anchor Display › Size) is passed to the tools.

Test anchors_are_picked_before_the_segments_through_them:

• A press 4.3 px from a corner and 0.5 px from its edge picks the corner. It picked the edge on main.
• At 10 px it picks the segment, and at 5 px it is out of reach.
• With Size 7 it reaches 6 px, with Tolerance 8 it reaches 7.5 px, and at 200% the reach scales.

Checked live on a curved path at 236%:

• A Direct Selection press-drag 4.2 screen px from the middle anchor moves the anchor with its handles.
• At 7 px it still grabs and bends the curve.

Checks

• Based on origin/main c3b73f0.
• cargo test -p vectorcraft-tools -p vectorcraft-engine -p vectorcraft-ui-egui.
• cargo clippy --all-targets -- -D warnings for those three crates.
• cargo fmt --all, cargo xtask wasm.

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