e-stepper: arrows start at the tag's centre and come down onto objects

[martin-henz]

Follow-up to #152 (program references).

• The arrow now starts at the centre of the tag (the #n badge or the frame bracket's label), not its right edge.
• Arrows to a function or a list end at the horizontal centre of its top, and come down onto it from above in both layouts (side by side and stacked). Arrows to a frame still end at the frame's left edge.
• The plugin's own diagram reports those anchors; the frontend's lent view does the same in source-academy/frontend (branch e-stepper-arrow-anchors).

Tests: arrow geometry (arriving from above), the start point, and the diagram's anchors against the layout.

🤖 Generated with Claude Code

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