Mutation→context merge primitive across the stack

The @client(merge=[context, ...]) decorator lets a mutation patch its
return value directly into the cached context bundle by matching the
mutation's Output type against each context-function's Output type
to identify the slot, then splicing server-side. Kernel runs
splice_slot on the response to apply locally — no refetch, no
invalidate-cascade.

Lands H14, H15, H16, M19, M20 from ISSUES.md.

Backends (Django + FastAPI):
  _resolve_merges() in both executors walks @client(merge=...) targets,
  resolves the per-context slot via types_match_for_merge, and emits
  {context, slot, value, params?} entries on the response. Param
  auto-scoping mirrors _resolve_invalidation's tier-1 logic.

Frontend kernel (mizan-base):
  Response handler reads the merge[] array and applies splice_slot
  for each entry — locates the cached context bundle by name+params,
  overwrites the named slot with the new value, notifies subscribers.

Core (mizan-python):
  @client decorator extended with merge= parameter. Schema export
  threads merge metadata onto the OpenAPI x-mizan-functions entries.

Examples / fixtures:
  fastapi-react-site harness exercises merge + Playwright spec covers
  the end-to-end happy path (mutation → instant UI update without
  network refetch). AFI fixture's rename_user function is the
  canonical merge target.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/afi/fixture.py

@@ -82,6 +82,18 @@ def update_profile(request, user_id: int, name: str) -> StatusOutput:
     return StatusOutput(ok=True)
 
 
+@client
+def find_user(request, user_id: int) -> ProfileOutput | None:
+    """Optional return — exercises Pydantic `T | None` schema introspection."""
+    return None
+
+
+@client(merge="user")
+def rename_user(request, user_id: int, name: str) -> ProfileOutput:
+    """Merge target — kernel splices return value into the user context."""
+    return ProfileOutput(user_id=user_id, name=name)
+
+
 # ─── Registration ───────────────────────────────────────────────────────────
 
 
@@ -92,3 +104,5 @@ def register_fixture() -> None:
     register(user_profile, "user_profile")
     register(user_orders, "user_orders")
     register(update_profile, "update_profile")
+    register(find_user, "find_user")
+    register(rename_user, "rename_user")