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/pipe-fitting/tool.tsx

@@ -1,7 +1,13 @@
 'use client'
 
 import { emitter, type GridEvent, PipeFittingNode, useScene } from '@pascal-app/core'
-import { CursorSphere, EDITOR_LAYER, triggerSFX, useEditor } from '@pascal-app/editor'
+import {
+  CursorSphere,
+  EDITOR_LAYER,
+  isGridSnapActive,
+  triggerSFX,
+  useEditor,
+} from '@pascal-app/editor'
 import { useViewer } from '@pascal-app/viewer'
 import { Html } from '@react-three/drei'
 import { useEffect, useMemo, useRef, useState } from 'react'
@@ -141,7 +147,7 @@ const PipeFittingTool = () => {
         resolvePlacement(
           raw,
           previewNode,
-          useEditor.getState().gridSnapStep,
+          isGridSnapActive() ? useEditor.getState().gridSnapStep : 0,
           manualQuatRef.current,
         ),
       )
@@ -157,7 +163,7 @@ const PipeFittingTool = () => {
       const { position, rotation } = resolvePlacement(
         lastRawRef.current,
         previewNode,
-        useEditor.getState().gridSnapStep,
+        isGridSnapActive() ? useEditor.getState().gridSnapStep : 0,
         manualQuatRef.current,
       )
       const fitting = PipeFittingNode.parse({