Make generated constraints compile without raw todo placeholders

[blackopsrepl]

Summary

Generated constraint skeletons currently contain raw todo!() placeholders in executable paths.

Problem

Newly generated constraint code is unfinished by default and can panic or fail to compile cleanly.

Why This Matters

Scaffolded code should feel intentional and should guide first-time users instead of dropping them into unsafe placeholders.

Proposed Work

• Replace raw todo!() calls with compile-safe placeholders, guided unimplemented!() usage, or commented examples that keep projects buildable.
• Improve post-generation instructions so users know what to fill in.

Acceptance Criteria

• Generated constraints are clearer and safer for end users.
• The generated code path is intentionally designed, documented, and tested.

Source

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

[blackopsrepl]

Resolved by #18.

We replaced raw generated constraint placeholders with compile-safe fail-fast guidance, kept pair/join scaffolds wired to the planning-variable join key, and extended generator coverage so the generated code stays buildable.