feat(#76): IMP-47B reject-as-AI-adaptation activation (u1~u13 backend + tests)

- u1~u9: AI fallback infrastructure (router/prompts/schema/validator) + Step 12 hook
- u10: e2e reject chain (writes final.html with AI-repaired slot, full coverage)
- u11: frontend wiring deferred to follow-up commit (split from IMP-41 hunks)
- u12: coverage_invariant guard
- u13: cache save gate (visual_check PASS + user_approved/auto_cache) — Codex #22 verified

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_phase_z2_ai_fallback_config.py

@@ -44,3 +44,43 @@ def test_ai_fallback_budget_and_circuit_defaults_locked() -> None:
     s = Settings()
     assert s.ai_fallback_budget_per_run == 10
     assert s.ai_fallback_circuit_breaker_threshold == 5
+
+
+# IMP-46 u5 — auto-cache opt-in setting default lock.
+# The CLI flag ``--auto-cache`` in src/phase_z2_pipeline.py mutates this
+# setting at parse time. The default MUST stay OFF so the dual-gate
+# contract (visual_check_passed AND user_approved) survives without an
+# explicit operator opt-in.
+
+
+def test_ai_fallback_auto_cache_default_off() -> None:
+    s = Settings()
+    assert s.ai_fallback_auto_cache is False, (
+        "IMP-46 u5 auto-cache MUST default OFF; the dual-gate contract "
+        "(visual_check_passed AND user_approved) survives without an "
+        "explicit --auto-cache opt-in."
+    )
+
+
+# IMP-47B u1 — reject route hint policy correction.
+# Prior to 2026-05-21 the reject V4 label routed to ``design_reference_only``
+# (no AI). The user policy correction (issue #76) reroutes reject to
+# ``ai_adaptation_required`` so the rank-1 reject frame is kept and the AI
+# re-maps MDX content into its declared slots. Activation remains gated by
+# ``ai_fallback_enabled`` (default OFF preserves the normal-path AI=0
+# contract — see test_ai_fallback_master_flag_default_off above).
+
+
+def test_reject_route_hint_routes_to_ai_adaptation() -> None:
+    from src.phase_z2_pipeline import _IMP05_ROUTE_HINTS, _imp05_route_hint
+
+    assert _IMP05_ROUTE_HINTS["reject"] == "ai_adaptation_required", (
+        "IMP-47B u1: reject must route to ai_adaptation_required so the "
+        "rank-1 reject frame is retained and AI re-maps MDX content into "
+        "its slots (frame auto-swap forbidden)."
+    )
+    assert _imp05_route_hint("reject") == "ai_adaptation_required"
+    # Sibling routes unchanged — guardrail against accidental drift.
+    assert _imp05_route_hint("use_as_is") == "direct_render"
+    assert _imp05_route_hint("light_edit") == "deterministic_minor_adjustment"
+    assert _imp05_route_hint("restructure") == "ai_adaptation_required"