refactor(phase0): source-agnostic formatter/gating scaffold + harness (inert) (#28)

Foundation for making all hazard formatting+gating source-agnostic. ZERO
behavior change — the formatter/decider registries are empty (get_formatter/
get_decider return None → existing precomposed/Mode-B path preserved), and the
shadow comparator is off unless MESHAI_SHADOW_CATEGORIES is set.

- notifications/formatters/ (registry+dispatch with family fallback), gating/
  (GateResult + deferred-commit contract), both empty registries.
- notifications/clock.py determinism seam; route wfigs/quake/nws gating time
  reads through it (identical values) so goldens can freeze time.
- formatters/_budget.py = copy of central/budget.py; central/budget.py is now a
  re-export shim (import-smoke test guards it).
- compose_mesh_message consults the registry first (verbatim, no Mode-B re-cap),
  falls back to legacy; _resolve_budget injects per-category budget.
- notifications/shadow.py + two DRY-RUN hooks (consumer._normalize, dispatcher
  render): compute the new result and diff-log SHADOW_MISMATCH JSONL, but NEVER
  commit/emit/write tables and always broadcast the OLD result. Inert by default.
- tests/harness (pinned_time/pinned_tz, byte-golden, gate-sequence) +
  scripts/capture_fixtures.py (ephemeral read-only NATS capture); tzdata pinned.

Tests: +60 (18 scaffold + 42 harness/shadow); 0 new failures (34 baseline).

Co-authored-by: Matt Johnson <mj@k7zvx.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

work/tests/test_import_smoke.py

@@ -0,0 +1,38 @@
+"""Phase-0 import smoke test: every meshai/central/*_handler.py module
+must be importable without error after the budget shim refactor.
+
+This guards against a broken import chain (e.g. circular imports or a bad
+re-export in the shim) that would silently break all handlers.
+"""
+
+import glob
+import importlib
+import os
+
+
+def _handler_modules():
+    """Collect meshai.central.*_handler module names by globbing the source tree."""
+    pattern = os.path.join(
+        os.path.dirname(__file__),
+        "..", "meshai", "central", "*_handler.py",
+    )
+    paths = sorted(glob.glob(pattern))
+    assert paths, "No *_handler.py files found — check the glob path"
+    modules = []
+    for p in paths:
+        name = os.path.basename(p)[:-3]  # strip .py
+        modules.append(f"meshai.central.{name}")
+    return modules
+
+
+def test_all_central_handlers_importable():
+    """Each *_handler module must import cleanly (no ImportError / circular deps)."""
+    failed = []
+    for mod_name in _handler_modules():
+        try:
+            importlib.import_module(mod_name)
+        except ImportError as exc:
+            failed.append(f"{mod_name}: {exc}")
+    assert not failed, (
+        "The following handler modules raised ImportError:\n" + "\n".join(failed)
+    )