Function createInstancedLinesMesh

Create an instanced mesh for lines rendering.

Sets up the instanced geometry with the texture-backed per-segment storage pair (RGBA32F line texture + aSortedIndex) attached to a shared unit-quad base.

Note: We use THREE.Mesh instead of THREE.InstancedMesh because:

• InstancedMesh adds instanceMatrix (mat4 = 4 attribute locations)
• Our shader computes positions from custom attributes, not matrices
• This avoids exceeding WebGL's 16 attribute location limit
• InstancedBufferGeometry with Mesh still uses instanced drawing