refactor(erd): extract ErdViewController and ErdCanvas from _ErdViewState #1690

Follow-up to #1362, which took erd_view.dart from 2,895 to 1,705 lines (layout engine, router and geometry in core/erd, ErdTableCard, ErdRelationPainter, ErdExporter, note and group widgets, search / views / export menus). What is left is one class, _ErdViewState: about 1,580 lines, ~135 members, ~31 state fields, and a 305-line build.

What is still in _ErdViewState

• Canvas: scale, pan, fit (_setScale, _zoomBy, _fit), tap and hover (_tapCanvas, _hoverCanvas), the InteractiveViewer in build.
• Selection and focus: _selectTable, _clearSelection, _syncFocus, _cardFocusFor, search picking.
• Saved views: _applyState, _showState, _switchView, _newView, _renameView, _deleteView, _scheduleSave, _saveNow, _current.
• Groups and notes: create, edit, gather, drag, resize.
• Card dragging and re-routing: _dragStart/_dragMove/_dragEnd, _scheduleRoutes.

Change

1. ErdViewController (a ChangeNotifier, no widget imports) owns selection, focus, search state, saved views, groups and notes. The pure operations that already live in core/erd (erd_group_ops, erd_note_ops, erd_saved_layout) stay there and are called from it.
2. ErdCanvas widget owns the InteractiveViewer, scale / fit / tap / hover and the drag of cards.
3. _ErdViewState composes the two with the toolbar, menus and dialogs.

Acceptance

• erd_view.dart is at most ~900 lines, and build is split into parts of a readable size.
• Selection, focus, saved-view switching, groups and notes are covered by unit tests that need no WidgetTester.
• The existing 52 widget tests under test/features/erd pass unchanged (no loosened expectations).
• No change in behaviour or look; hover and drag rebuild only what they rebuilt before (perf(erd): hover rebuilds and repaints the whole canvas; no viewport culling; first layout on the UI thread #1169, perf(erd): router rebuilds the obstacle grid and re-routes every edge on each drag move #1168).

Not in scope

• The minimap from refactor(erd): decompose 2900-line ErdView into pure ErdLayoutEngine, ErdCanvas, ErdTableCard and ErdMinimap #1362: there is no minimap in the product.
• The "under 300 lines" target of refactor(erd): decompose 2900-line ErdView into pure ErdLayoutEngine, ErdCanvas, ErdTableCard and ErdMinimap #1362: it no longer describes how the view is built.

Notes

Medium risk (31 fields shared by most handlers); do it as a few small PRs: controller first, canvas second. Low priority unless ERD work is planned.