fix(2-G.5): preview_for_settings contract in adapter docstring + distinguish [] from None

Fixup 1 — Contract section appended to SourceAdapter.preview_for_settings's
docstring. Override authors read adapter.py, not routes.py, so the contract
(pure function of settings; open your own short-lived aiohttp session; None
vs [] semantics) belongs on the base method, not on the GUI stub class.

Fixup 2 — _adapter_preview.html distinguishes [] from None. Previously the
elif test was truthiness (`elif preview_rows`) which collapsed both into
"render nothing". Now uses `elif preview_rows is not none` and special-cases
the empty-list case inside: legend "Preview (0 rows)" with no table; None
still renders nothing at all. Lets adapters signal "query ran, matched zero"
distinctly from "preview not meaningful".

Tests +1:
- test_partial_renders_empty_list — [] yields "Preview (0 rows)" legend,
  no table, no headers. Distinct from the existing None case.

Acceptance:
- 27/27 targeted (preview_hook +1 new, nwis, stream_registry).
- 458/458 full suite.
- (b) framework GUI dir still has zero adapter-name branches.

src/central/adapter.py

@@ -85,5 +85,16 @@ class SourceAdapter(ABC):
         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.
+
+        Contract:
+        - Preview is a pure function of `settings`. Do NOT access
+          self._config_store or cursor_db state — the framework may instantiate
+          adapters with a stub config_store solely to call this method.
+        - Network preview implementations must open their own short-lived
+          aiohttp session (the adapter's polling session may not exist; the GUI
+          process never calls startup()).
+        - Return None when preview is not meaningful (e.g., required settings
+          like region are unset). Return [] explicitly if the query ran and
+          matched zero rows — the framework renders that distinctly from None.
         """
         return None

src/central/gui/templates/_adapter_preview.html

@@ -2,9 +2,10 @@
 <article aria-label="Preview Unavailable" style="background-color: var(--pico-del-color); margin-bottom: 1rem;">
     <strong>{{ preview_error }}</strong>
 </article>
-{% elif preview_rows %}
+{% elif preview_rows is not none %}
 <fieldset>
     <legend>Preview ({{ preview_rows|length }} rows)</legend>
+    {% if preview_rows %}
     <table class="preview-table" role="grid">
         <thead>
             <tr>
@@ -19,5 +20,6 @@
             {% endfor %}
         </tbody>
     </table>
+    {% endif %}
 </fieldset>
 {% endif %}