The cable bus block currently doesn't occlude light as I'd expect it to.

For example, looking up to a cable that should fully occlude the sun will instead look like the same light level as if it was not present:

This means that cable facades can not effectively block light, making cables a liability in mob spawners and such

Some pointers:
Our block properties for CableBusBlock specify noOcclusion() which is obviously wrong if we want to occlude 😓

We override propagatesSkylightDown:
Which is also likely broken since it overrides the default method from Block which actually takes the shape into account.
@Override public boolean propagatesSkylightDown(BlockState state, BlockGetter reader, BlockPos pos) { return true; }

We might need to updateLight if a part or facade is changed since it would affect the light-level computation and we do not change the block state in such cases.

This is what Vanilla Levels do when a block state changes:

if ((flags & 128) == 0
	&& blockState2 != blockState
	&& (
		blockState2.getLightBlock(this, pos) != blockState.getLightBlock(this, pos)
			|| blockState2.getLightEmission() != blockState.getLightEmission()
			|| blockState2.useShapeForLightOcclusion()
			|| blockState.useShapeForLightOcclusion()
	)) {
	this.getProfiler().push("queueCheckLight");
	this.getChunkSource().getLightEngine().checkBlock(pos);
	this.getProfiler().pop();
}

So we'd likely to have to call this.getChunkSource().getLightEngine().checkBlock(pos);.

A related issue is culling faces of neighboring blocks. The logic in net.minecraft.world.level.block.Block#shouldRenderFace completely skips detecting if a side of any block is occluded by the neighbor (the cable bus), if the neighbor has set noOcclusion() (like we do). That will propagate further into getFaceOcclusionShape(), which will delegate to getShape() normally. We could optimize this though by detecting if a facade is present and just returning a full block for that side.