Problem

@use currently resolves only stdlib modules. There is no mechanism for user-defined or third-party packages with versioning, no lockfile integration for external deps, and no path toward an ecosystem.

Acceptance Criteria

• Defined package manifest format (e.g. nls.toml or nlspackage.nl)
• @use package_name@version syntax works for local and registered packages
• Lockfile captures external dependency hashes
• nlsc install or equivalent resolves and pins dependencies
• Does not require the full package registry (that's v1.0 feat: ANLU package registry #27)

Why This Blocks v0.5

Ecosystem depth requires a package model. Without it, users are permanently limited to stdlib.