Domain isolation: typed contracts, service registry, and composable wiring

Add typed service contracts (Protocols + frozen DTOs) in shared/contracts/
for cross-domain communication. Each domain exposes a service interface
(BlogService, CalendarService, MarketService, CartService) backed by SQL
implementations in shared/services/. A singleton registry with has() guards
enables composable startup — apps register their own domain service and
stubs for absent domains.

Absorbs glue layer: navigation, relationships, event handlers (login,
container, order) now live in shared/ with has()-guarded service calls.
Factory gains domain_services_fn parameter for per-app service registration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

contracts/__init__.py

@@ -0,0 +1,31 @@
+"""Typed contracts (DTOs + Protocols) for cross-domain service interfaces."""
+
+from .dtos import (
+    PostDTO,
+    CalendarDTO,
+    CalendarEntryDTO,
+    MarketPlaceDTO,
+    ProductDTO,
+    CartItemDTO,
+    CartSummaryDTO,
+)
+from .protocols import (
+    BlogService,
+    CalendarService,
+    MarketService,
+    CartService,
+)
+
+__all__ = [
+    "PostDTO",
+    "CalendarDTO",
+    "CalendarEntryDTO",
+    "MarketPlaceDTO",
+    "ProductDTO",
+    "CartItemDTO",
+    "CartSummaryDTO",
+    "BlogService",
+    "CalendarService",
+    "MarketService",
+    "CartService",
+]