feat(stacks): add stackDiscovery.component.defaultConfig hook

[terlar]

Adds a per-component function option that is called for every discovered component and returns a plain attrset of component options merged before component.nix.

Receives { componentPath, stackName, componentName } so consumers can derive filesystem-based defaults without re-scanning or re-implementing the discovery logic.

Semantics:

• Freeform attributes: merged before component.nix, so component.nix wins via ordering. Do not wrap in lib.mkDefault — types.attrs does not unwrap priority wrappers.
• Declared options: wrap individual values in lib.mkDefault to allow component.nix to override without a priority conflict.
• Default: _: {} (no-op).