Pin cost-algebra moat vs Dynamo-style block scoring

[fxdv]

Summary

• Add CI-gated Dynamo foil contrast tests: unbounded/NaN overlap can undercut healthy workers or erase prefill; Demiurge fail-expensive + ρ_max cannot.
• Wire new tests into DEMI-FAIL-EXPENSIVE / DEMI-WARM-DISCOUNT in the requirements registry.
• Document the competitive failure-mode table in PRODUCT-AND-DESIGN (honest: Dynamo remains a fair Track D baseline on honest telemetry).

Test plan

• I have read CLA.md and certify my contribution under Apache-2.0 OR MIT

• cargo test -p demiurge-cost --test dynamo_contrast --test positivity

• cargo xtask lint

• CI Gate (Verify / Track A / Spec)