fix(engine): validate complete flashback costs

[nishu-builder]

Summary

Checks the whole nonmana flashback cost before advertising a graveyard cast. This keeps spells such as Group Project out of legal actions when the player cannot tap enough creatures, while preserving payable flashback casts and other graveyard-cast permissions.

Files changed

• crates/engine/src/game/casting.rs
• crates/engine/tests/integration/flashback_nonmana_payability.rs
• crates/engine/tests/integration/main.rs

CR references

• CR 702.34a — flashback uses its printed alternative cost.
• CR 118.3 — every resource required by a cost must be available.
• CR 601.2f — the selected alternative cost is part of the total cost.
• CR 601.2h — a cost cannot be paid partially.

Implementation method (required)

Method: /engine-implementer

Track

Developer

LLM

Model: codex-5
Thinking: high
Tier: Standard

Verification

• Required checks ran clean, or the exact CI-owned alternative is stated below.

• Gate A output below is for the current committed head.

• Final review-impl below is clean for the current committed head.

• Both anchors cite existing analogous code at the same seam.

• cargo fmt --all -- --check — passed.

• cargo test -p engine --test integration flashback_nonmana_payability -- --nocapture — 3 passed, 0 failed.

• cargo clippy-strict — passed for the workspace and all targets.

• cargo test -p engine — 16,554 unit tests and 3,012 integration tests passed.

• ./scripts/gen-card-data.sh — generated and validated 35,397 card faces; 31,851 of 34,626 cards fully implemented.

• cargo coverage — 31,346 of 35,397 cards supported (88.6%).

• cargo semantic-audit — 32,417 cards audited; 294 existing findings.

• Tilt was unavailable in this checkout, so the documented direct Developer-track fallback was used.

Gate A

Gate A PASS head=9ccaf70f7aebe19e7193d27c42c25c6d07a4d218 base=8e424583e9167324bdcd77667a6bee0469ebfe70

Anchored on

• crates/engine/src/game/casting.rs:1032 — Escape delegates its complete residual cost to AbilityCost::is_payable.
• crates/engine/src/game/casting.rs:9572 — Evoke checks the complete nonmana residual before offering the variant.

Final review-impl

Final review-impl PASS head=9ccaf70f7aebe19e7193d27c42c25c6d07a4d218

Claimed parse impact

None.

Validation Failures

Initial ./scripts/setup.sh --agent did not complete its optional WASM bootstrap when the shared disk filled. The casting-stack-conditions reference check also reports two stale function-name anchors already absent on upstream (pay_and_push and pay_ability_cost); all other references and every required gate above passed.

CI Failures

None.

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[github-actions]

Parse changes introduced by this PR

✓ No card-parse changes detected.

[matthewevans]

Approved — complete non-mana flashback costs now gate legal actions at the prepared-cast authority.

✅ Clean

• crates/engine/src/game/casting.rs:12039-12046 delegates to AbilityCost::is_payable, the established recursive non-mana cost gate already used by the Escape and Evoke prechecks in this module.
• crates/engine/tests/integration/flashback_nonmana_payability.rs:82-96 reaches the legal-action surface for both the unpayable and payable Group Project cases; :101-117 drives payment and resolution; :125-150 keeps Festival of Embers' independent graveyard permission available.
• crates/engine/tests/integration/main.rs:129 registers the integration module. The current-head parse-diff reports no card-parse changes, and the required Rust and Frontend checks are green.

Recommendation: approve and enqueue.