fix(engine): make the "before attackers declared" window phase-based (CR 508.1/508.2)

[Spaceint]

Summary

is_before_attackers_declared (the shared predicate behind the BeforeAttackersDeclared casting and activation restrictions) gated the window on priority_seat(state) == state.active_player. That clause is only ever satisfiable by the active player, so it wrongly closed the "before attackers are declared" window for non-active actors.

Per CR 508.1 ("First, the active player declares attackers. This turn-based action doesn't use the stack") + CR 508.2 ("Second, the active player gets priority"), attackers are declared as a turn-based action before any player receives priority. So "before attackers are declared" is a pure phase property — the PreCombatMain | BeginCombat run-up — independent of who currently holds priority. This PR drops the priority-seat clause and keeps the phase check.

What was broken (and is now fixed)

• Master Warcraft — "Cast this spell only before attackers are declared." A non-active player could never cast it during another player's combat.
• Siren's Call — "…only during an opponent's turn, before attackers are declared." Its two casting restrictions were mutually exclusive under the old clause (DuringOpponentsTurn needs active_player != caster, but the window needed the caster's seat == active player) → entirely uncastable front-door.

Why it's safe

26 of the 28 BeforeAttackersDeclared cards are [DuringYourTurn, BeforeAttackersDeclared] (Portal Three Kingdoms tap-ability cycle, King's Assassin, etc.). Restrictions are AND-composed, and DuringYourTurn => active_player == player independently pins those to the active player — so a phase-only window cannot widen them. The only two BeforeAttackersDeclared-without-DuringYourTurn cards (Master Warcraft, Siren's Call) are exactly the intended beneficiaries. No existing test exercises the old priority-gated behavior; turn-control (CR 723) can't affect a check that never reads priority.

Anchored on

• crates/engine/src/game/restrictions.rs — is_before_combat_damage / is_sorcery_speed_window: sibling timing predicates that key on state.phase (phase-based window pattern).
• crates/engine/src/game/restrictions.rs — ActivationRestriction::DuringYourTurn => state.active_player == player: the AND-composed active-player gate that keeps the 26 turn-qualified cards pinned.

Test Plan

Three runtime restriction-check tests drive the production entry points (check_casting_restrictions / check_activation_restrictions):

• before_attackers_window_admits_non_active_caster — a non-active player is inside the window in both PreCombatMain and BeginCombat (with an active-player reach-guard). Fail-on-revert measured: restoring the priority-seat clause makes only this test fail.
• before_attackers_window_closes_at_and_after_declaration — window closed at DeclareAttackers/DeclareBlockers/PostCombatMain, open at BeginCombat (reach-guard).
• during_your_turn_before_attackers_stays_active_player_gated — [DuringYourTurn, BeforeAttackersDeclared] stays legal on the controller's own turn / illegal on an opponent's turn, across both phases (non-regression).

Verification (Non-developer track; Tilt down → direct cargo)

• cargo test -p engine — 196 test binaries, 0 failures
• CI-parity clippy (--workspace --exclude phase-tauri --all-targets --features engine/proptest -- -D warnings) — exit 0
• cargo check -p engine-wasm --target wasm32-unknown-unknown — exit 0
• cargo fmt --all — clean
• Gate A (./scripts/check-parser-combinators.sh) — exit 0 (no parser change)

Model: claude-opus-4-8
Thinking: high
Tier: Frontier
Track: Non-developer (card-data.json integration deferred to CI per docs/AI-CONTRIBUTOR.md §2)

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[github-actions]

Parse changes introduced by this PR

✓ No card-parse changes detected.

[matthewevans]

Approved. I checked the restriction seam and CR boundary: BeforeAttackersDeclared is enforced in the shared casting/activation restriction predicate, and CR 508.1/508.2 make this a phase boundary before declare-attackers priority, not an active-player priority-seat check.

The added tests are discriminating: the non-active caster case fails on the old priority-seat gate, the declare-attackers/post-combat phases stay closed, and [DuringYourTurn, BeforeAttackersDeclared] activations remain active-player gated.