Entity-aware select/move tool and smarter snapping

[lewish]

What

Reworks the select tool to be entity-aware and makes snapping rebuild connections cleanly when shapes move or are drawn.

Select tool

• Move a whole box — grab a box's empty interior and drag. Border + contents move, and attached lines stay connected and reflow: straight runs shorten/lengthen, perpendicular moves bend, the connector is traced through corners to its real terminal, and arrows pointing into the box ride along.
• Resize a box edge — grab the border (unchanged DrawMove).
• Reshape a line tip — drag a free tip and it behaves like the line tool (free 2D, corners), pivoting at the nearest bend; bend orientation follows the segment's own axis (Ctrl/Shift flips).
• Move a word — grab a run of text. Shift-click to build a multi-selection.
• Rubber-band selections reflow lines that cross the selection edge.
• Selection travels with undo/redo and is purged when leaving the tool.

Snapping

• Accumulate connections so a cell that gains/loses several at once lands on the right junction (no more last-write-wins leaving a side disconnected).
• Complete the disconnect table for ┼ (DOWN/LEFT/RIGHT) so drawing a line onto another yields a T, not a +.
• Normalisation pass — after a change, each affected line/junction is tidied to match its neighbours (a dangling ┤ → │, a stray ┼ → the right T, a moved box's connection points settle correctly). Scoped to never mutate the content being moved, only the structure around it.
• Boxes snap to adjacent lines/boxes when drawn, like lines already did.

Testing

New specs cover the happy paths and every regression fixed here: entity.spec, select.spec, snap.spec, box.spec, line.spec, grid.spec (incl. a 2x2-grid quadrant move). All client + common suites pass.

🤖 Generated with Claude Code