Pure constraint math built on the registry's `MovableConfig`:
- `resolveMovable(node)` — reads `def.capabilities.movable`, runs the
optional `override(ctx)` callback (returning null falls back to the
base config). Returns null when the kind isn't movable.
- `applyAxisLock(current, target, axes)` — projects 3D motion onto
the allowed axes; locked components fall back to current.
- `moveToward(node, current, target, options?)` — top-level helper
combining axis lock + (optional) grid snap. Returns null when the
node is not movable.
- `movePlanToward(node, currentY, current, target, options?)` —
X/Z-plane convenience for floor/plan-view placement.
- `isMovable(node)` — predicate for tools/UI gating.
Tests cover override callback, null-override fallback, axis lock
permutations, grid-snap on/off, and the 2D plan convenience.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
