Add `tier2_enabled: bool` field to `Metadata` schema (epic #150 Phase 1.6)

[dackclup]

Context

Tracker issue for epic #150 Phase 1.6 — the deferred-fix item carried forward from closed issue #117 (verify-helper Section B post-PR-#79 stale expectations, closed by PR #149).

Issue #117's acceptance criteria item 4 reads:

Add a regression-guard sanity check — IF _EIGHT_K_DEFENSES_ENABLED ever flips back to False at compute time, fires SHOULD be 0; the helper should read the enabled-flag state from the compute output (e.g., a tier2_enabled: bool metadata field if available, else inferred from tier2_coverage_pct)

PR #149 shipped the soft-warning-band Section B fix but deferred the schema-side tier2_enabled surfacing for a separate PR (would have required a schema bump → out-of-scope for the verify-helper-only PR). This issue is that deferred work, now tracked explicitly.

Why a dedicated field

Today verify-production-output/helper.py infers Tier-2 enablement implicitly from tier2_coverage_pct > 0. That works only while at least one 8-K defense is wired ON — if a future PR flips compute/scoring/tier2._EIGHT_K_DEFENSES_ENABLED = False (e.g., emergency disable during an EDGAR outage), the helper would silently expect 0 fires AND get 0 fires, masking the disablement event in Section A-H output.

An explicit tier2_enabled: bool field in Metadata:

1. Lets the verifier print "Tier-2 8-K defenses: DISABLED (compute-time flag)" as a top-line state, not a derived inference
2. Makes the compute-time flag self-documenting in frontend/public/data/metadata.json (auditable from the static site without grepping source)
3. Pairs with the regression-guard logic from PR fix(verify-output): Section B Tier-2 post-PR-#79 stale expectations (closes #117) #149 Section B (helper reads flag → expected fires conditional on flag)

Acceptance criteria

• Add tier2_enabled: bool to compute/output/schemas.py::Metadata (default True for back-compat with old snapshots)
• Source value from compute/scoring/tier2._EIGHT_K_DEFENSES_ENABLED at writer time in compute/output/writer.py
• Mirror in frontend/lib/types.ts::Metadata
• Regenerate frontend/lib/schema-snapshot.json (python -m compute.output.schema_check --update-snapshot)
• Update verify-production-output/helper.py Section B to read metadata["tier2_enabled"] and short-circuit expected-fires logic when False
• Tests:
  • Add 1 case to tests/test_output_writer.py (or similar) asserting tier2_enabled round-trips correctly
  • Add 1 case to the upcoming tests/test_verify_helper.py (epic Foundation reconciliation roadmap — process + analytical scrutinize findings (2 rounds) #150 Phase 1.4) exercising the tier2_enabled=False path
• Bump schema version (current 0.9.2-phase4h.2 → next; coordinate with epic Foundation reconciliation roadmap — process + analytical scrutinize findings (2 rounds) #150 Phase 2's planned 0.10.0 bump so we don't burn two version numbers)
• CLAUDE.md + AGENTS.md lockstep diff

Coordination notes

• Epic #150 Phase 3.4 also lists "Implement tier2_enabled: bool metadata field (from 1.6)" — this issue is the tracker for the work; the implementation PR can land under either Phase 1 (cheap-and-cheerful) or Phase 3 (bundled with structural changes). Owner's discretion.
• If implemented in Phase 1 (before Phase 2's 0.9.2 → 0.10.0 bump), use a minor patch version (e.g., 0.9.3-phase4h.3) to avoid a wasted major schema bump.
• If implemented in Phase 3, fold into the 0.10.0 schema bump alongside the valuation_methods_applicable: int field from Phase 2.1.

Out of scope

• Generalizing to per-flag enablement metadata ({flag_name: bool} map) — overkill for a single feature flag; revisit if more compute-time feature flags appear
• Frontend UI surfacing — purely metadata; the static site doesn't need to render an "8-K defenses: DISABLED" banner

Severity

Low — informational hygiene + observability. The current implicit inference works correctly under all currently-shipped compute paths; this is forward-looking robustness for the regression case where a future PR disables Tier-2.

---

Filed via Claude Code closing out epic #150 Phase 1.6.