LayerNode graph should fail with a named error when a dependency is undefined

Description

When a layer node's deps array (or a LayerNode.group item) contains undefined, the layer graph walker crashes with:

TypeError: undefined is not an object (evaluating 'node.name')

The stack points into Effect internals rather than the broken node, which makes this hard to diagnose. I hit it in a compiled build where a module export was undefined because of a runtime import cycle; finding the offending node required temporarily patching the walker.

Request

Validate dependencies in LayerNode.make and LayerNode.group and fail with an error that names the owner node and the dependency index, for example:

LayerNode @opencode/v2/FileSystem: invalid dependency at index 2 (undefined)

This is a diagnosability guard; the existing behavior for valid graphs is unchanged.

Search the TypeScript sources for LayerNode.make, LayerNode.group, and the layer graph walker. Trace how dependency entries are read, then add validation that reports the owner node and dependency index for undefined entries while preserving valid-graph behavior. Verify the resulting error includes the requested diagnostic details.