Fix classic co-op auto-shot death animation ordering

[xcomcoopdev]

Summary

Fixes a classic-turn co-op regression where the client could fire the remaining rounds of an auto shot before displaying the target unit's collapse animation.

The client could consequently show later rounds hitting a unit that had already fallen on the host, then change the target directly from standing to dead after the burst.

Cause

The receive-order barrier treated the classic unit_death packet as unable to overtake its pending hit_unit opener. Both packets could remain behind the active projectile gate while the subject-less hasHitUnit packet was still allowed through.

This released ExplosionBState and allowed ProjectileFlyBState to create the next round before the client had queued its authoritative UnitDieBState.

In addition, allowing after_unit_death through as a chain closer could apply the final dead status before the queued client death animation started. UnitDieBState would then see an already-out unit and skip its collapse frames.

Changes

• Keep the opener-order barrier restricted to parallel turns, preserving the original classic packet-closing behavior.
• Allow classic unit_death to queue the authoritative client UnitDieBState while the shot is waiting.
• Stop treating classic after_unit_death as a gate-bypassing chain closer, so it cannot apply the final dead status ahead of the collapse animation.
• Keep unit_death releasing the parked explosion immediately after the death state has been queued.
• Expose fallPhase through the test harness battle state for animation-order assertions.
• Add a classic co-op auto-shot regression test using a rifle with finite ammunition.

The parallel unit_casualty and death-ghost paths are unchanged.

Testing

Added:

python tools/coop_test/test_classic_autoshot_death_pacing.py

The test stages a deterministic lethal first round in a three-round auto shot and verifies that the client enters intermediate collapse frames while two rounds still remain.

RED result without the fix:

[FAIL] client displayed no intermediate UnitDieBState collapse frame: [(0, 0, 19), (0, 0, 18), (0, 0, 17), (6, 0, 17)]

GREEN result with the fix:

PASS: Classic client collapsed the victim before consuming the remaining auto-shot rounds (ammo 20 -> 19 during collapse -> 17).

The corrected behavior was also verified manually in classic turns.