feat(engine): enter under an opponent’s control on ETB (CR 110.2a)

[galuis116]

Summary

Models the self-ETB replacement " enters under the control of an opponent of your choice" — the permanent enters the battlefield directly under an opponent’s control (CR 110.2a), never under its owner’s first. Previously the clause was dropped (Effect:effect_structure gap) and the permanent entered under its owner’s control.

Unlocks (each was unsupported on this single gap): Xantcha, Sleeper Agent; Captive Audience; Pendant of Prosperity; Abby, Merciless Soldier.

Completes the self-replacement side of the enters_under work whose imperative/engine half landed in #2817 (any-player ControllerRef resolution, CR 110.2a).

Approach (parameterize, don’t proliferate)

• types/ability.rs — ReplacementDefinition gains enters_under: Option<ControllerRef> (serde default/skip_serializing_if — card-data back-compatible), parallel to the imperative Effect::ChangeZone.enters_under slot. No new enum variant.
• game/replacement.rs — surfaced as an EventModifiers field and stamped onto the entering ZoneChange’s controller_override where etb_tap_state is applied — before ETB triggers fire. resolve_self_enters_under_controller resolves Opponent against the entering object’s controller (the canonical resolver returns None for ambiguous Opponent): sole opponent in two-player, first in seat order otherwise.
• parser/oracle_replacement.rs — parse_self_enters_under_opponent emits the Moved/SelfRef/Battlefield/enters_under=Opponent replacement (nom combinators; self-subject gate covers ~ incl. legendary short names, and this <card-type>).

Notes

“of your choice” is deterministic in two-player (the engine’s primary mode / CI). A full multiplayer choice is a follow-up; CR 110.2a holds either way.

Tests

• self_enters_under_opponent_parses_controller_override_replacement (4-phrasing build-the-class)
• external_enters_under_opponent_is_not_a_self_replacement (self-subject gate)
• self_enters_under_opponent_replacement_routes_control_to_opponent (end-to-end: enters under the opponent’s control via the real ChangeZone pipeline)

Parser-combinator gate and rustfmt --check clean.

Fixes #2834

[gemini-code-assist]

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[matthewevans]

Approving — this is exactly how to complete a cross-PR mechanic. It slots cleanly into the seams #2817 established and is well-tested at the layer that matters.

Verified:

• Parameterization is textbook. ReplacementDefinition::enters_under: Option<ControllerRef> (not a bool), serde default/skip_serializing_if so card-data stays back-compatible, with a builder — mirroring the imperative Effect::ChangeZone.enters_under slot from #2817. None = enters under owner (every existing replacement is unaffected).
• Zone-pipeline integration is correct. The override resolves to a concrete PlayerId and is written onto the ZoneChange's existing controller_override during the replacement step, so the permanent enters under the opponent's control directly — never under its owner first (CR 614.1c, verified; CR 110.2a "unless the effect states otherwise," verified). Reusing the imperative path's controller_override slot rather than inventing a parallel mechanism is the right call.
• Tests are strong. self_enters_under_opponent_replacement_routes_control_to_opponent drives the real ChangeZone pipeline and asserts controller == opponent (the discriminating runtime check for #2917's class of cards), plus a parser test and a negative control (external_enters_under_opponent_is_not_a_self_replacement) that pins the self-vs-external boundary.
• Class-level. One shared self-replacement gap unlocks Xantcha, Captive Audience, Pendant of Prosperity, and Abby — all previously entering under the owner's control, fully broken.

On the multiplayer note: resolving ControllerRef::Opponent to the first opponent in seat order is consistent with the rest of the engine — controller_ref_player returns None for Opponent everywhere (filter.rs:659), so "an opponent of your choice" prompting is a genuinely cross-cutting follow-up, not a corner cut here. Two-player is fully correct, and multiplayer still enters under an opponent (satisfying CR 110.2a's core). Worth a tracking issue for the chosen-opponent prompt across the enters_under feature, but it shouldn't hold this. Enqueuing.