fix(engine): fire ETB observers when an as-enters-choice permanent enters (#830)

[matthewevans]

Summary

Fixes #830 — when a permanent enters carrying an "As it enters, choose …" replacement (e.g. Valgavoth's Lair → "choose a color"), the entry pauses on WaitingFor::NamedChoice instead of WaitingFor::Priority. The canonical priority-time trigger collection was skipped, so every ETB observer (constellation like Doomwake Giant, Soul Warden, etc.) was silently dropped for that entry.

Root cause

run_post_action_pipeline collects ETB triggers only on a WaitingFor::Priority result; an as-enters-choice entry returns NamedChoice, so the entering permanent's ZoneChanged never reached process_triggers.

Fix

Generalize the existing state.deferred_entry_events mechanism (previously gated to copy-target / counter choices) to capture as-enters NamedChoice { source_id: Some(_) } entries, and replay them on choice resolution through a shared replay_deferred_entry_events helper that surfaces interactive OrderTriggers/DistributeAmong pauses. CR 614.12a (the choice is made before the permanent enters).

Double-fire fix (engine_priority.rs)

The CastSpell path (creature/enchantment via stack resolution) does not skip run_post_action_pipeline the way PlayLand does, so an as-enters-choice creature's ETB observer was collected twice — once from the still-present entry ZoneChanged in events, once from the deferred replay. The trigger scan now excludes events already in deferred_entry_events while leaving them in events for frontend animation (a "two readers, one writer" contract). This also eliminates a pre-existing latent double-fire on the copy-target-choice path.

Class coverage

Engine-pipeline-level fix covering ~280 as-enters-choice permanents × the entire ETB-observer class — both PlayLand (lands) and CastSpell (creatures/enchantments) entry paths.

Tests (constellation_enters_with_choice.rs, 3/3)

• as_enters_choice_land_fires_constellation — land path repro (toughness 2→1; discriminates zero-fire AND double-fire).
• plain_enchantment_still_fires_constellation — regression guard for the untouched Priority path.
• as_enters_choice_creature_fires_soul_warden — creature-via-stack double-fire discriminator (life +1, not +2).

Plan reviewed clean. Implementation reviewed clean twice (the second review, after the double-fire fix, adversarially verified copy-path safety, hot-path perf N≈0, GameEvent PartialEq soundness, and double-fire completeness). Full test-engine suite green (12791 passed). CR numbers grep-verified.

[gemini-code-assist]

Code Review

This pull request addresses issue #830 where enters-the-battlefield (ETB) observers failed to trigger for permanents with 'As it enters, choose...' replacement effects. The solution consolidates deferred battlefield-entry event replay logic into a single helper, replay_deferred_entry_events, and extends mid-entry choice capture to support NamedChoice events. This ensures ETB triggers are deferred and replayed exactly once against the fully realized post-choice object. Comprehensive integration tests have been added to verify the fix and guard against regressions. No review comments were provided, so there is no feedback to address.

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