Auto-detect and initialise the best available kernel.
Tries brepjs-opencascade (OCCT) first, then falls back to brepkit-wasm.
For occt-wasm, use registerKernel directly (see tests/helpers/kernelInit.ts).
Idempotent — calling it again after a kernel is registered is a no-op that returns the current kernel ID immediately.
The kernel ID that was initialised ('occt' or 'brepkit').
brepjs — Public API