fix: remove dead throttled faction readback fallback in attack_system (#249)

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Summary

Round 1-4 authority.md compliance audit for issue #249.

Round 1 (Inventory): Cataloged all GpuReadState read sites, GPU write sites, buffer sizing constants, combat target validation paths, and EntityMap count/iteration usage.

Round 2 (Violation Detection): Found one code-quality issue: dead code fallback at combat.rs:485 referenced gpu_state.factions (throttled readback, 60-frame staleness) in an unreachable unwrap_or_else branch. The target_npc is always Some at that point (None/dead filtered above), so the fallback never executes — but having it reference throttled readback was misleading and could become a real violation if the flow changed.

All other rules pass: no feedback loops, no throttled readback as hard gate, all buffer sizing uses MAX_ENTITIES, EntityMap methods used for all type-specific counts.

Round 3 (Fix): Removed the dead fallback. Replaced with target_npc.map(|n| n.faction).unwrap_or(-1). Added regression test attack_system_uses_ecs_faction_not_gpu_readback that sets gpu_state.factions to stale same-faction data and verifies combat still fires (proving ECS faction from EntityMap is used, not GPU readback).

Round 4 (Compliance table): Posted as issue comment.

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/review findings

Check	Result
Linked issue	pass -- #249
Issue acceptance checklist	pass -- 5/5 checked in issue comment
PR checklist coverage	pass -- summary covers all 4 rounds, linked issue, tests
Associated docs	pass -- authority.md unchanged (no new patterns discovered)
authority.md	pass -- fix removes throttled readback fallback, enforces ECS authority
k8s.md	pass -- no entity architecture changes
performance.md	not required
Build	pass
Automated tests	pass -- 381/381
Regression tests	pass -- 1 new: attack_system_uses_ecs_faction_not_gpu_readback
Benchmarks	not required
BRP launch	not run
Issue-specific verification	pass -- compliance table posted on issue, all systems pass/fixed

Verdict: ready to merge

Findings

• No findings. Clean audit with one dead-code fix and proper regression test.

Acceptance coverage

• Round 1 inventory -> pass (issue comment with file:line refs)
• Round 2 violation detection -> pass (1 violation found, correctly identified)
• Round 3 fix -> pass (dead fallback removed, regression test added)
• Round 4 compliance table -> pass (15 systems audited, all pass or fixed)
• Compliance verified -> pass (all 3 docs checked)

Docs and policy coverage

• authority.md -> compliant (fix enforces Hard Rule Slice: Rest + Heal phase migration #1: never use throttled readback as validity gate)
• k8s.md -> compliant (no entity changes)
• performance.md -> not required

Regression coverage

• attack_system_uses_ecs_faction_not_gpu_readback: sets stale same-faction GPU data, verifies combat still fires using ECS faction from EntityMap