Add widget registry for universal UI decoupling

Introduces a widget system where domains register UI fragments into
named slots (container_nav, container_card, account_page, account_link).
Host apps iterate widgets generically without naming any domain.

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

contracts/widgets.py

@@ -0,0 +1,49 @@
+"""Widget descriptors for cross-domain UI composition.
+
+Each widget type describes a UI fragment that one domain contributes to
+another domain's page.  Host apps iterate widgets generically — they never
+name the contributing domain.
+"""
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Callable
+
+
+@dataclass(frozen=True, slots=True)
+class NavWidget:
+    """Renders nav items on a container page (entries, calendars, markets)."""
+    domain: str
+    order: int
+    context_fn: Callable   # async (session, *, container_type, container_id, **kw) -> dict
+    template: str
+
+
+@dataclass(frozen=True, slots=True)
+class CardWidget:
+    """Decorates content cards in listings with domain data."""
+    domain: str
+    order: int
+    batch_fn: Callable     # async (session, post_ids) -> dict[int, list]
+    context_key: str       # key injected into each post dict
+    template: str
+
+
+@dataclass(frozen=True, slots=True)
+class AccountPageWidget:
+    """Sub-page under /auth/<slug>/."""
+    domain: str
+    slug: str
+    label: str
+    order: int
+    context_fn: Callable   # async (session, *, user_id, **kw) -> dict
+    template: str
+
+
+@dataclass(frozen=True, slots=True)
+class AccountNavLink:
+    """Nav link on account page (internal or external)."""
+    label: str
+    order: int
+    href_fn: Callable      # () -> str
+    external: bool = False