docs(examples): expand hello_seller.py to full 9-method sales-non-guaranteed surface

The canonical example tripped validate_platform soft-warns for four
SalesPlatform Protocol methods required in v6.0 rc.1+: get_media_buys,
list_creative_formats, list_creatives, provide_performance_feedback. A
"hello world" example that emits warnings at boot teaches adopters to
ignore validator signal — the opposite of the fail-fast contract.

Add minimal stub implementations for all four methods, each clearly
annotated to wire to the adopter's inventory/analytics system in
production. Update module and class docstrings to correctly state the
9-method required surface rather than the stale "five required methods"
count. validate_platform now passes clean with zero soft-warns.

Closes #515

https://claude.ai/code/session_01SQEG3PCARK4eKHBbqu4fTS

Author: claude

examples/hello_seller.py

@@ -1,15 +1,18 @@
-"""Hello-seller — the smallest possible v6.0 DecisioningPlatform.
+"""Hello-seller — the canonical v6.0 DecisioningPlatform starting point.
 
-A minimal :class:`SalesPlatform` adopter showing the canonical surface:
+A minimal :class:`SalesPlatform` adopter showing the full required surface:
 
 * :class:`DecisioningCapabilities` declared on the class body
 * :class:`SingletonAccounts` for the dev/single-tenant case
-* Five required ``sales-non-guaranteed`` methods (``get_products``,
-  ``create_media_buy``, ``update_media_buy``, ``sync_creatives``,
-  ``get_media_buy_delivery``) — all sync, sync return path. The full
-  required set is enforced at server boot by ``validate_platform``
-  via :data:`REQUIRED_METHODS_PER_SPECIALISM` — omitting any of the
-  five fails fast with INVALID_REQUEST.
+* Nine required ``sales-non-guaranteed`` methods — five hard-required
+  (``get_products``, ``create_media_buy``, ``update_media_buy``,
+  ``sync_creatives``, ``get_media_buy_delivery``) plus four required by the
+  SalesPlatform Protocol for any ``sales-*`` specialism in v6.0 rc.1+
+  (``get_media_buys``, ``list_creative_formats``, ``list_creatives``,
+  ``provide_performance_feedback``). The authoritative source is the
+  :data:`~adcp.decisioning.dispatch.REQUIRED_METHODS_PER_SPECIALISM` and
+  :data:`~adcp.decisioning.dispatch.RECOMMENDED_METHODS_PER_SPECIALISM`
+  maps; ``validate_platform`` checks both at server boot.
 
 Run::
 
@@ -77,13 +80,18 @@
 
 
 class HelloSeller(DecisioningPlatform):
-    """The canonical minimal v6.0 sales-non-guaranteed adopter.
+    """The canonical v6.0 sales-non-guaranteed adopter.
 
-    Implements all five required methods of ``sales-non-guaranteed``
-    (the full contract per :data:`REQUIRED_METHODS_PER_SPECIALISM`):
-    ``get_products``, ``create_media_buy``, ``update_media_buy``,
-    ``sync_creatives``, ``get_media_buy_delivery``. ``validate_platform``
-    runs at boot and fails fast on any missing method.
+    Implements all nine required methods of ``sales-non-guaranteed``: the
+    five hard-required (``get_products``, ``create_media_buy``,
+    ``update_media_buy``, ``sync_creatives``, ``get_media_buy_delivery``)
+    and the four required by the SalesPlatform Protocol in v6.0 rc.1+
+    (``get_media_buys``, ``list_creative_formats``, ``list_creatives``,
+    ``provide_performance_feedback``).
+
+    ``validate_platform`` runs at boot and fails fast on any missing
+    hard-required method; it soft-warns (or hard-fails in strict mode) for
+    the four rc.1+ methods. This example passes all checks cleanly.
     """
 
     capabilities = DecisioningCapabilities(
@@ -234,6 +242,66 @@ def get_media_buy_delivery(
             ],
         }
 
+    # ---- v6.0 rc.1 required methods ----------------------------------------
+    # These four methods are required by the SalesPlatform Protocol for any
+    # sales-* specialism in v6.0 rc.1+. The stubs below return the minimal
+    # valid wire shape (empty collections / acknowledged). Wire them to your
+    # inventory system / analytics pipeline in production.
+
+    def get_media_buys(
+        self,
+        req: Any,
+        ctx: RequestContext[Any],
+    ) -> dict[str, Any]:
+        """List media buys for the resolved account.
+
+        Return all active media buys here. Wire to your order-management
+        system in production; buyers use this for status polling and
+        reconciliation.
+        """
+        return {"media_buys": []}
+
+    def list_creative_formats(
+        self,
+        req: Any,
+        ctx: RequestContext[Any],
+    ) -> dict[str, Any]:
+        """Catalog of accepted creative formats.
+
+        Return the full list of format definitions this seller accepts.
+        Wire to your creative-spec registry in production; buyers use this
+        to validate creatives before submission.
+        """
+        return {"formats": []}
+
+    def list_creatives(
+        self,
+        req: Any,
+        ctx: RequestContext[Any],
+    ) -> dict[str, Any]:
+        """List the seller's view of buyer-uploaded creatives.
+
+        Return all creatives the buyer has synced. Wire to your creative
+        asset store in production; buyers use this to check approval
+        statuses and discover available creatives.
+        """
+        return {"creatives": []}
+
+    def provide_performance_feedback(
+        self,
+        req: Any,
+        ctx: RequestContext[Any],
+    ) -> dict[str, Any]:
+        """Buyer-supplied performance signal back to the seller.
+
+        Acknowledge receipt and log to your analytics pipeline in
+        production. Buyers send conversion events (clicks, installs,
+        purchases) here so the seller can optimize pacing and targeting.
+        """
+        return {"success": True}
+
+    # -------------------------------------------------------------------------
+
     @staticmethod
     def _get_packages(req: Any) -> list[dict[str, Any]]:
         """Pull the wire ``packages`` array from the request, tolerating