Lock segment molding to an axis with Shift

[Chetansahney]

Closes #3760

While molding a path segment with the Path tool, holding Shift now constrains the drag to a single axis (horizontal or vertical), matching the direction-locking behavior of other drag operations in the tool.

Behavior

• Hold Shift during a segment mold to lock the movement to the dominant screen axis (whichever of X/Y the drag has travelled further along).
• Release Shift to resume free movement.
• A "Lock Direction" hint is shown in the status bar while molding.

Implementation

• mold_handle_positions gains a lock_direction flag; when set, the incoming pointer position is snapped to the grab point's x or y before the drag delta is computed, so the segment moves purely along one axis.
• The Path tool forwards the existing snap_angle (Shift) key state into the molding branch, where it was previously unused, and adds the corresponding hint.

[gemini-code-assist]

Code Review

This pull request introduces a "Lock Direction" feature when molding segments in the path tool. By holding the modifier key (Shift), the drag is constrained to the dominant screen axis (purely horizontal or vertical). Dynamic hints have also been updated to display this option. There are no review comments, so I have no feedback to provide.

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[0HyperCube]

The implementation in #3766 is far more complete. It features overlays so you can see where you are aligning to (in your implementation the user just has to guess). It also does not offset the cursor position in such a strange way.