feat(2-G.5): preview_for_settings framework hook + NWIS opt-in

Adds an optional async hook on SourceAdapter so any adapter can surface a
settings-driven preview on its /adapters/<name> edit page. The framework
renders the result generically as a table — no adapter-name branches in
GUI templates or route code.

Framework changes:
- src/central/adapter.py: new async preview_for_settings(self, settings)
  on the base class, default returns None. Adapters opt in by overriding;
  non-overriding adapters render unchanged.
- src/central/gui/routes.py: GET /adapters/{name} instantiates the adapter
  with a no-op _PreviewConfigStore stub and a /dev/null cursor path (GUI
  has no live ConfigStore), constructs settings_obj via the schema, and
  calls preview_for_settings inside a try/except. Result lands in template
  context as preview_rows / preview_error.
- src/central/gui/templates/_adapter_preview.html: new partial. Generic
  table with columns derived from the first dict's keys; error banner
  mirrors the existing last_error article style.
- src/central/gui/templates/adapters_edit.html: one-line include between
  the Region fieldset and Save/Cancel.

NWIS opt-in:
- New NWIS_MONITORING_LOCATIONS_URL constant and _PREVIEW_LIMIT cap of 50.
- preview_for_settings returns None when region is None, otherwise one-shot
  fetches monitoring-locations within the bbox via a fresh aiohttp session.
  Must work even when adapter is not started -- the GUI process never calls
  startup(). Returns list[dict] with the contract column order: site_id,
  name, site_type, state. Errors propagate so the framework can render the
  operator-visible banner.
- HTTP call factored into _fetch_preview_text so tests mock cleanly.

Tests (7 new):
- tests/test_preview_hook.py: default returns None; partial renders list
  with correct headers/rows/count; partial renders error banner; partial
  renders empty when both context values are None.
- tests/test_nwis.py adds TestNWISPreview: returns None without region,
  returns rows with correct column order, propagates HTTP errors.

Verification:
- 457/457 full suite green (was 450; +7 new tests).
- Live /adapters/nwis preview returns 50 rows with the contract keys
  against the current production Iowa bbox.
- /adapters/eonet preview_for_settings returns None via base default --
  proves framework is duck-typed, no NWIS-specific code in framework.

tests/test_nwis.py

@@ -276,3 +276,84 @@ class TestNWISAdapter:
 
         assert first_pass
         assert second_pass == []
+
+
+class TestNWISPreview:
+    """Preview hook (PR G.5) — exercised without starting the adapter."""
+
+    @pytest.mark.asyncio
+    async def test_preview_returns_none_without_region(self, tmp_path: Path):
+        from central.adapters.nwis import NWISAdapter, NWISSettings
+
+        adapter = NWISAdapter(_config({}), MagicMock(), tmp_path / "cursors.db")
+        result = await adapter.preview_for_settings(NWISSettings())
+        assert result is None
+
+    @pytest.mark.asyncio
+    async def test_preview_returns_rows_with_region(self, tmp_path: Path):
+        """Given a bbox, preview returns one dict per monitoring-locations feature
+        with the contract column order: site_id, name, site_type, state."""
+        from central.adapters.nwis import NWISAdapter, NWISSettings
+
+        sample_response = json.dumps({
+            "type": "FeatureCollection",
+            "features": [
+                {
+                    "type": "Feature",
+                    "id": "USGS-05420500",
+                    "geometry": {"type": "Point", "coordinates": [-90.25, 41.78]},
+                    "properties": {
+                        "monitoring_location_name": "MISSISSIPPI RIVER AT CLINTON, IA",
+                        "site_type_code": "ST",
+                        "state_name": "Iowa",
+                    },
+                },
+                {
+                    "type": "Feature",
+                    "id": "USGS-05454500",
+                    "geometry": {"type": "Point", "coordinates": [-91.65, 41.66]},
+                    "properties": {
+                        "monitoring_location_name": "IOWA RIVER AT IOWA CITY, IA",
+                        "site_type_code": "ST",
+                        "state_name": "Iowa",
+                    },
+                },
+            ],
+        })
+
+        adapter = NWISAdapter(
+            _config({"region": {"west": -94.0, "south": 40.0, "east": -93.0, "north": 41.0}}),
+            MagicMock(),
+            tmp_path / "cursors.db",
+        )
+        adapter._fetch_preview_text = AsyncMock(return_value=sample_response)
+        settings = NWISSettings(region=adapter.region)
+        rows = await adapter.preview_for_settings(settings)
+
+        assert rows is not None
+        assert len(rows) == 2
+        # Column order is part of the contract — first row's keys must match exactly.
+        expected_keys = ["site_id", "name", "site_type", "state"]
+        assert list(rows[0].keys()) == expected_keys
+        # Row content reflects fixture data.
+        assert rows[0]["site_id"] == "USGS-05420500"
+        assert rows[0]["name"] == "MISSISSIPPI RIVER AT CLINTON, IA"
+        assert rows[0]["site_type"] == "ST"
+        assert rows[0]["state"] == "Iowa"
+        assert rows[1]["site_id"] == "USGS-05454500"
+
+    @pytest.mark.asyncio
+    async def test_preview_propagates_http_error(self, tmp_path: Path):
+        """Preview must not swallow upstream errors — the framework needs them
+        to render the operator-visible 'Preview unavailable: …' banner."""
+        from central.adapters.nwis import NWISAdapter, NWISSettings
+
+        adapter = NWISAdapter(
+            _config({"region": {"west": -94.0, "south": 40.0, "east": -93.0, "north": 41.0}}),
+            MagicMock(),
+            tmp_path / "cursors.db",
+        )
+        adapter._fetch_preview_text = AsyncMock(side_effect=RuntimeError("upstream 503"))
+        settings = NWISSettings(region=adapter.region)
+        with pytest.raises(RuntimeError, match="upstream 503"):
+            await adapter.preview_for_settings(settings)