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.

src/central/adapter.py

@@ -78,3 +78,12 @@ class SourceAdapter(ABC):
     async def shutdown(self) -> None:
         """Optional lifecycle hook called on graceful shutdown."""
         pass
+
+    async def preview_for_settings(self, settings: BaseModel) -> list[dict] | None:
+        """Optional. Override to surface a settings-driven preview on the edit page.
+
+        Return list[dict] (framework renders as a generic table; columns come from
+        the first dict's keys, in insertion order). Return None to skip preview.
+        Raise to surface an error banner — framework catches at the route boundary.
+        """
+        return None

src/central/adapters/nwis.py

@@ -28,6 +28,12 @@ logger = logging.getLogger(__name__)
 NWIS_LATEST_CONTINUOUS_URL = (
     "https://api.waterdata.usgs.gov/ogcapi/v0/collections/latest-continuous/items"
 )
+NWIS_MONITORING_LOCATIONS_URL = (
+    "https://api.waterdata.usgs.gov/ogcapi/v0/collections/monitoring-locations/items"
+)
+# Per-render cap for the settings-driven preview (PR G.5). Keep small so the
+# /adapters/<name> edit page renders quickly.
+_PREVIEW_LIMIT = 50
 
 # Single source of truth for the parameter-code default. Operators tune via
 # NWISSettings.parameter_codes; do NOT duplicate this list elsewhere
@@ -375,3 +381,55 @@ class NWISAdapter(SourceAdapter):
             geo=Geo(centroid=centroid),
             data=data,
         )
+
+    async def _fetch_preview_text(self, url: str) -> str:
+        """One-shot GET for the preview render.
+
+        Uses a fresh aiohttp session — preview must work even when the adapter
+        isn't started (the GUI process never calls startup()). Factored out so
+        tests can mock the HTTP call without touching aiohttp internals.
+        """
+        async with aiohttp.ClientSession(
+            timeout=aiohttp.ClientTimeout(total=15),
+        ) as session:
+            async with session.get(
+                url, headers={"User-Agent": "Central/0.4"}
+            ) as resp:
+                resp.raise_for_status()
+                return await resp.text()
+
+    async def preview_for_settings(self, settings: NWISSettings) -> list[dict] | None:
+        """Surface monitoring-locations inside the configured bbox.
+
+        Returns up to _PREVIEW_LIMIT rows from the monitoring-locations
+        collection. Returns None if region is unset (no useful preview).
+        Raises on HTTP / JSON / shape failure — framework catches at the route.
+        """
+        if settings.region is None:
+            return None
+
+        params = {
+            "bbox": (
+                f"{settings.region.west},{settings.region.south},"
+                f"{settings.region.east},{settings.region.north}"
+            ),
+            "limit": str(_PREVIEW_LIMIT),
+        }
+        url = f"{NWIS_MONITORING_LOCATIONS_URL}?{urlencode(params)}"
+
+        text = await self._fetch_preview_text(url)
+        page = json.loads(text)
+        features = page.get("features") or []
+
+        rows: list[dict] = []
+        for feat in features:
+            props = feat.get("properties") or {}
+            rows.append(
+                {
+                    "site_id": feat.get("id"),
+                    "name": props.get("monitoring_location_name"),
+                    "site_type": props.get("site_type_code"),
+                    "state": props.get("state_name"),
+                }
+            )
+        return rows

src/central/gui/routes.py

@@ -46,6 +46,9 @@ from central.gui.audit import (
 )
 from functools import cache
 
+from pathlib import Path
+
+from central.config_models import AdapterConfig
 from central.gui.db import get_pool
 from central.gui.form_descriptors import describe_fields, FieldDescriptor
 from central.adapter_discovery import discover_adapters
@@ -55,13 +58,23 @@ from pydantic import ValidationError
 @cache
 def _adapter_classes() -> dict:
     """Cached adapter class discovery.
-    
+
     GUI is a separate process from supervisor; walks pkgutil itself.
     Python's import cache makes subsequent calls free.
     """
     return discover_adapters()
 
 
+class _PreviewConfigStore:
+    """No-op stand-in passed to adapter __init__ when calling preview_for_settings.
+
+    preview_for_settings implementations must create their own one-shot HTTP
+    session and must not depend on config_store / cursor_db state — the GUI
+    process has no live ConfigStore (the supervisor owns the real one)."""
+
+    pass
+
+
 router = APIRouter()
 
 # Streams to display on dashboard -- derived from the registry's dashboard flag.
@@ -1450,6 +1463,28 @@ async def adapters_edit_form(
             )
             api_key_missing = not has_key
 
+    # Generic settings-driven preview. Adapters opt in by overriding
+    # SourceAdapter.preview_for_settings; the framework is duck-typed on the
+    # returned list[dict] shape and never branches on adapter name.
+    preview_rows: list[dict] | None = None
+    preview_error: str | None = None
+    if adapter_cls is not None and hasattr(adapter_cls, "settings_schema"):
+        try:
+            settings_obj = adapter_cls.settings_schema(**settings)
+            preview_cfg = AdapterConfig(
+                name=row["name"],
+                enabled=row["enabled"],
+                cadence_s=row["cadence_s"],
+                settings=settings,
+                updated_at=row["updated_at"],
+            )
+            preview_adapter = adapter_cls(
+                preview_cfg, _PreviewConfigStore(), Path("/dev/null")
+            )
+            preview_rows = await preview_adapter.preview_for_settings(settings_obj)
+        except Exception as exc:
+            preview_error = f"Preview unavailable: {exc}"
+
     csrf_token = request.state.csrf_token
     response = templates.TemplateResponse(
         request=request,
@@ -1466,6 +1501,8 @@ async def adapters_edit_form(
             "tile_attribution": tile_attribution,
             "api_key_missing": api_key_missing,
             "requires_api_key_alias": requires_api_key_alias,
+            "preview_rows": preview_rows,
+            "preview_error": preview_error,
         },
     )
     return response

src/central/gui/templates/_adapter_preview.html

@@ -0,0 +1,23 @@
+{% if preview_error %}
+<article aria-label="Preview Unavailable" style="background-color: var(--pico-del-color); margin-bottom: 1rem;">
+    <strong>{{ preview_error }}</strong>
+</article>
+{% elif preview_rows %}
+<fieldset>
+    <legend>Preview ({{ preview_rows|length }} rows)</legend>
+    <table class="preview-table" role="grid">
+        <thead>
+            <tr>
+                {% for col in preview_rows[0].keys() %}<th>{{ col }}</th>{% endfor %}
+            </tr>
+        </thead>
+        <tbody>
+            {% for row in preview_rows %}
+            <tr>
+                {% for col in preview_rows[0].keys() %}<td>{{ row[col] }}</td>{% endfor %}
+            </tr>
+            {% endfor %}
+        </tbody>
+    </table>
+</fieldset>
+{% endif %}

src/central/gui/templates/adapters_edit.html

@@ -178,6 +178,8 @@
     </fieldset>
     {% endif %}
 
+    {% include "_adapter_preview.html" %}
+
     <button type="submit">Save Changes</button>
     <a href="/adapters" role="button" class="outline">Cancel</a>
 </form>