docs: add detailed explanation of three edge rendering forms and operation points

[Copilot]

The basic edge tutorial lacked any explanation of how the three built-in edge types differ in rendering and what interactive handles each provides.

Changes

Added two new sections to tutorial/basic/edge.zh.md and tutorial/basic/edge.en.md:

Three Edge Rendering Forms

Covers the SVG primitive used, path computation strategy, and click-area expansion approach for each type:

Edge type	SVG element	Path computation
line	<line>	Direct start→end, no calculation
polyline	<polyline>	A* + Manhattan distance, orthogonal routing around nodes
bezier	<path> (M…C…)	Auto-computed cubic Bézier control points (sNext / ePre)

Operation Point Differences

Explains the interactive handles available when an edge is selected:

Handle type	line	polyline	bezier
Start/end adjust points	✓	✓	✓
Per-segment drag (H/V)	✗	✓	✗
Bézier control points (sNext / ePre)	✗	✗	✓

Key behavioral notes documented:

• line: path is immutable; only endpoints can be reconnected
• polyline: horizontal segments drag vertically (ns-resize), vertical segments drag horizontally (ew-resize); adjustEdgeMiddle restricts dragging to middle segments only
• bezier: selecting the edge reveals two independent control-point handles with guide lines via BezierAdjustOverlay; dragging either changes the curve shape on that side independently

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