fix(engine): evaluate 'attacks alone' relative to the attacker, not the ability source

[dhgoal]

Tier: Frontier
Model: claude-opus-4-8
Thinking: high

Closes #5328.

Summary

The "attacks alone" mechanic silently failed to fire when the observing permanent stayed back and a different creature attacked alone.

TriggerCondition::MinCoAttackers tallied co-attackers by excluding the ability source (source_id). "Attacks alone" is encoded as Not(MinCoAttackers { minimum: 1 }). When a non-attacking observer (an Exalted-style card — the issue's Agent 13, Sharon Carter / Black Widow Double Agent) is the ability source but is not in combat, excluding source_id from the tally is a no-op — so the lone attacker stays counted, MinCoAttackers returns true, and Not(...) reports "not alone." The ability never fires.

Fix: exclude the creature that actually fired the trigger, not the ability source. The per-attacker AttackersDeclared event (produced by matching_attack_events, and re-checked at resolution per CR 603.4) names exactly the triggering attacker; use it, falling back to source_id when the event names no single attacker (self-referential trigger paths — behavior unchanged). source_id still resolves the filter's source object, so Training's power-relative co-attacker comparison (CR 702.149a) is unaffected.

Reproduction (confirmed failing-before / passing-after)

A four-board probe isolates exactly one wrong quadrant on main:

board	expected	on main
observer stays back, 1 other attacks alone	fires (1 clue)	0 clues — BUG
observer stays back, 2 others attack	silent (0)	0 ✓
observer attacks alone	fires (1)	1 ✓
observer + 1 other attack	silent (0)	0 ✓

The new test other_creature_attacks_alone_fires_even_when_observer_stays_back is the regression pin (0 before this change, 1 after); two_other_creatures_attacking_do_not_fire_attacks_alone is the negative control.

Note for the maintainer: the reporter also mentioned possible over-triggering with multiple attackers; I could not reproduce that on any board (the multi-attacker quadrant is already correct, above). The confirmed defect is the under-triggering case named in the issue title ("not triggering"), which this fixes. Flagging the discrepancy transparently rather than claiming to cover the parenthetical.

Anchored on

• crates/engine/src/game/triggers.rs — the TriggerCondition::MinCoAttackers arm in check_trigger_condition: the co-attacker filter previously read a.object_id != source_id, now excludes the event's triggering attacker.
• The AttackersDeclared { attacker_ids, .. } event (from matching_attack_events, re-passed at resolution-time re-check per CR 603.4) — the source of the single triggering-attacker id.

Verification

• cargo fmt --all — clean.
• cargo clippy -p engine --tests -- -D warnings — clean.
• No parser change (engine-only) → parser combinator gate N/A.
• cargo test -p engine --test attacks_alone_observer_5328 — 2/2 pass (fail-on-revert).
• cargo test -p engine --lib -- game::triggers:: — 353 passed; full integration suite — 1823 passed. Pre-existing related tests still green: issue_5159_attacks_alone_investigate, training_condition_counts_only_higher_power_coattackers, granted_ability_self_binding.

CR verified against docs/MagicCompRules.txt: 506.5, 508.1a, 603.2c, 603.4, 702.83b, 702.149a.

Scope Expansion

None.

[superagent-security]

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[gemini-code-assist]

Code Review

This pull request adds a new integration test file attacks_alone_observer_5328.rs to verify the "attacks alone" mechanic (Issue #5328). The tests ensure that the mechanic is evaluated relative to the attacking creature rather than the ability's source, verifying that the trigger fires correctly when a different creature attacks alone and does not fire when multiple creatures attack. There are no review comments, so I have no feedback to provide.

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[github-actions]

Parse changes introduced by this PR

✓ No card-parse changes detected.