Undefined layer node dependency fails with an unattributable TypeError

Description

When a layer node's dependency array contains undefined, the graph compiler fails with an error that identifies nothing:

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

It comes from resolve in packages/core/src/effect/layer-node.ts:

{ cache, resolve: (node) => replacementMap.get(node.name) ?? node }

node is the undefined entry. In a release build the stack is minified and names no module, and the server reports a generic Unexpected server error.

A dependency ends up undefined when a circular import means the providing module hasn't finished evaluating at the point the deps: [...] array is built. Nothing validates the array, so the failure appears later and far from its cause.

This is what made #48372 hard to pin down: every prompt failed, the surfaced error looked like an auth/provider problem, and several people there suspected their credentials first.

Suggested fix: validate dependencies where the array is built and throw an error naming the node and the offending index, e.g.

Layer node "@opencode/v2/FileSystem" has an undefined dependency at index 2.

That is enough to locate the cycle immediately.

OpenCode version

1.18.30

Steps to reproduce

1. Create a circular import between two modules where one module's deps: [...] reads Other.node from the other.
2. Build a bundle whose entry causes the dependency module to evaluate first.
3. Run it — the dependency array captures undefined and compilation dies with the a.name TypeError.

filesystem.ts / filesystem/search.ts is a real instance (fixed separately in #48397); this issue is only about the missing validation and the resulting undiagnosable error.