refactor: more baseline code between node and edge shader getters, and create common attach for the 2d/3d skeleton render layers

[seankmartin]

Medium priority — done:

• defineShaderBase() method extracted into RenderHelper (L806) — handles the shared error check, defineCommonShader,
  defineAttributeAccess, defineDynamicSegmentAppearance, and the pick uniforms (uPickInstanceStride, vPickID,
  vSegmentValue)
• finalizeShaderBuilder() method extracted into RenderHelper (L832) — handles COLORMAPS, segment attr defines, user
  attr varying loop, setVertexMain, addControlsToBuilder, setFragmentMainFunction; the TODO about Uint32/Uint64 moved
  here as it's the right home
• Both edgeShaderGetter and nodeShaderGetter defineShader callbacks now open with defineShaderBase and close with
  finalizeShaderBuilder, leaving only primitive-specific code between them
• attachSpatiallyIndexedSkeletonLayer() free function extracted (L3170) — "2d" vs "3d" is the only argument that
  differs; both attach() methods now delegate in 2 lines

Low priority — done:

• PackedSkeletonGeometry.lod field removed; SpatiallyIndexedSkeletonChunk.lod field and its assignment also removed
• data: any → data: unknown in both updateMouseState methods (cast at the delegation site)
• position → _position in both getValueAt methods
• registerDisposer(base.redrawNeeded...) moved to immediately after registerDisposer(base) in the SliceView
  constructor, consistent with the Perspective constructor