fix(engine): run the base chain when a teamwork-instead swap doesn't fire

[tryeverything24]

Summary

Fixes #4772. Too Evil to Stay Dead ("Choose target creature card in your graveyard with
mana value 4 or less. If this spell was cast using teamwork, instead choose target creature
card in your graveyard. Return the chosen card to the battlefield.") silently did nothing
when cast without paying the optional Teamwork additional cost — mana was spent, the
spell resolved, but the targeted creature never returned to the battlefield.

Root cause

The not-swapped path for three of the four "instead" condition kinds
(AdditionalCostPaidInstead, CastVariantPaidInstead, TargetHasKeywordInstead) only ever
checked sub.else_ability, returning early when it was None — silently dropping
sub.sub_ability (here, the trailing ChangeZone that returns the card to the battlefield)
entirely. The sibling ConditionInstead arm already had the correct
sub.sub_ability tail-runner fallback (added for Throw from the Saddle / Evil's Thrall /
Take the Fall / That's Rough Buddy) — this merges all four condition kinds onto that single,
already-tested path, since the swap-decision check just above already evaluates all four
uniformly.

Anchored on

• crates/engine/src/game/effects/mod.rs:7830-7921 (pre-fix) — the ConditionInstead
  not-swap tail-runner this change generalizes, with its own existing regression coverage
  (condition_instead_not_swap_tail_runner_honors_gates,
  condition_instead_runs_base_chain_when_not_met).
• crates/engine/src/game/effects/mod.rs:6444-6473 — the swap-decision block, which already
  evaluates AdditionalCostPaidInstead/CastVariantPaidInstead/TargetHasKeywordInstead/
  ConditionInstead symmetrically, establishing that the not-swap path should be symmetric
  too.

Testing

Added crates/engine/tests/integration/issue_4772_too_evil_to_stay_dead.rs:

• too_evil_to_stay_dead_without_teamwork_still_reanimates — the actual regression (fails
  on pre-fix code with the exact reported symptom).
• too_evil_to_stay_dead_with_teamwork_still_reanimates — differential control, confirms
  the already-working swap path doesn't regress.

Verified the new test is load-bearing by reverting just the effects/mod.rs change and
re-running — it fails with the exact issue #4772 symptom, then passes again with the fix
restored. Both tests pass on a clean, isolated cargo test -p engine --test integration issue_4772 run (not relying on any shared/incremental build cache).

running 2 tests
test issue_4772_too_evil_to_stay_dead::too_evil_to_stay_dead_without_teamwork_still_reanimates ... ok
test issue_4772_too_evil_to_stay_dead::too_evil_to_stay_dead_with_teamwork_still_reanimates ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 3051 filtered out

cargo fmt --all run, no diff beyond the change above.

Validation Failures (honesty disclosure, per docs/AI-CONTRIBUTOR.md)

This was implemented and verified by a Claude Code agent (diagnosis via real code reading

• empirical test-driven verification, minimal fix reusing an existing tested code path,
  regression test proven load-bearing by revert/restore, cargo fmt clean), but not
  through the formal /engine-implementer multi-agent plan → review-plan → implement →
  review-impl pipeline — that skill wasn't available in the environment this was built in.
  No Gate A combinator-purity script output or discriminating-test coverage map /
  maintainer-simulation matrix is included. Flagging this plainly rather than claiming a
  review loop that didn't run.

Model: claude-sonnet-5
Tier: Standard

[gemini-code-assist]

Code Review

This pull request resolves issue #4772 where spells with 'instead' overrides (such as Teamwork) would silently drop their trailing effects when cast without paying the additional cost. The fix consolidates the handling of all four 'instead' condition kinds (AdditionalCostPaidInstead, CastVariantPaidInstead, TargetHasKeywordInstead, and ConditionInstead) in resolve_chain_body so they all correctly fall back to executing the base continuation chain when the condition is not met. Additionally, comprehensive integration tests have been added to verify both the teamwork and non-teamwork branches for 'Too Evil to Stay Dead'. I have no feedback to provide as there are no review comments.

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[github-actions]

Parse changes introduced by this PR

✓ No card-parse changes detected.

[matthewevans]

Verdict: Approved

🔴 Blocker

• None.

🟡 Non-blocking

• None.

✅ Clean

• resolve_chain_body is the shared non-swap authority for all four instead condition kinds; the rebased code retains main's continuation-context capture.
• The registered integration test drives both declined and paid Teamwork through casting, target selection, and stack resolution; the decline case fails if the new tail runner is removed.
• The current-head CI is green, Gemini reported no findings, and the parse-diff confirms no parser coverage change.

Recommendation: approve and enqueue.