feat(editor): MEP placement migration — Shift=cycle / mode-driven snapping

Migrate all 9 MEP kinds' placement tools onto the unified snapping model:
declare snapProfile ('item' for point-placed hvac-equipment / duct-terminal /
duct-fitting / pipe-fitting / pipe-trap; 'structural' for directional runs
duct-segment / pipe-segment / liquid-line / lineset), and replace the legacy
shiftKey-bypass reads with mode-driven isGridSnapActive / isMagneticSnapActive /
isAngleSnapActive. For runs the 45° lock becomes the cyclable 'angles' mode;
Alt stays the vertical-riser modifier (run drafting has no validity gate to
force). Port mating gated on "mode != off". Dropped stale "⇧ smooth/free" hints.

The bespoke MEP move-tool/selection (endpoint) tools stay on the legacy model —
they use setMovingNode(null) so no moving-scope context resolves yet; migrating
them needs scope-wiring first (follow-up).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/nodes/src/lineset/definition.ts

@@ -22,6 +22,10 @@ export const linesetDefinition: NodeDefinition<typeof LinesetNode> = {
   schema: LinesetNode,
   category: 'utility',
   distributionRole: 'run',
+  // Directional run: like a wall, drafting sets a direction, so it takes the
+  // structural snapping context (grid / lines / angles / off) with a 45° angle
+  // lock available as a cyclable mode.
+  snapProfile: 'structural',
 
   defaults: () => ({
     object: 'node',