Replace scaffolded data-loader stubs with compile-safe templates

[blackopsrepl]

Summary

Domain generation currently rewrites src/data/mod.rs with a todo!("Implement data loading") stub.

Problem

Generated projects are pushed away from compile-and-run quality by a known placeholder in a core path.

Why This Matters

Generated projects should compile cleanly when possible, or fail in a deliberate and well-explained way.

Proposed Work

• Generate a compile-safe placeholder implementation for the data-loading path.
• Add clear comments indicating where domain-specific loading logic belongs.
• Add scaffold smoke coverage for this generated path.

Acceptance Criteria

• Generated projects compile cleanly after domain generation, or failures are deliberate, obvious, and clearly explained.
• Automated scaffold tests cover the generated data module.

Source

Derived from PR #3 (PRODUCTION_REVIEW_ISSUES_2026-03-20.md).

[blackopsrepl]

Resolved by #19.

We replaced the generated data-loader todo! panic with a compile-safe placeholder implementation, exposed the rewrite path to direct tests, and covered the default scaffold cleanup path in CLI tests.