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>

backends/mizan-fastapi/tests/test_dispatch.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import asyncio
+
 import pytest
 from fastapi import FastAPI
 from fastapi.exceptions import RequestValidationError
@@ -34,6 +36,11 @@ class UserOutput(BaseModel):
     authenticated: bool
 
 
+class ItemOutput(BaseModel):
+    id: int
+    name: str
+
+
 @pytest.fixture
 def app():
     """Build a fresh FastAPI app + Mizan router with a few @client functions."""
@@ -63,12 +70,39 @@ def app():
     def whoami(request) -> UserOutput:
         return UserOutput(email="real@example.com", authenticated=True)
 
+    @client
+    def list_items(request) -> list[ItemOutput]:
+        return [ItemOutput(id=1, name="a"), ItemOutput(id=2, name="b")]
+
+    @client
+    def find_item(request, item_id: int) -> ItemOutput | None:
+        return ItemOutput(id=item_id, name="found") if item_id > 0 else None
+
+    @client(merge="items")
+    def set_item_name(request, id: int, name: str) -> ItemOutput:
+        return ItemOutput(id=id, name=name)
+
+    @client(context="items")
+    def items_list(request) -> list[ItemOutput]:
+        return [ItemOutput(id=1, name="orig")]
+
+    @client
+    async def async_echo(request, text: str) -> EchoOutput:
+        # await something on the loop to prove we're really running async
+        await asyncio.sleep(0)
+        return EchoOutput(message=f"async: {text}")
+
     register(echo, "echo")
     register(add, "add")
     register(current_user, "current_user")
     register(user_count, "user_count")
     register(update_email, "update_email")
     register(whoami, "whoami")
+    register(list_items, "list_items")
+    register(find_item, "find_item")
+    register(set_item_name, "set_item_name")
+    register(items_list, "items_list")
+    register(async_echo, "async_echo")
 
     fastapi_app = FastAPI()
     fastapi_app.include_router(mizan_router, prefix="/api/mizan")
@@ -171,3 +205,58 @@ class InvalidationTests:
         body = r.json()
         # affects='user' is a context-name string → invalidate list contains 'user'
         assert "user" in body["invalidate"]
+
+
+# ─── Structured-output shapes ───────────────────────────────────────────────
+
+
+class StructuredOutputTests:
+    """list[BaseModel] and Optional[BaseModel] should reach the wire as bare values, not {result: ...}."""
+
+    def test_list_of_basemodel_returns_bare_array(self, http):
+        r = http.post("/api/mizan/call/", json={"fn": "list_items", "args": {}})
+        assert r.status_code == 200
+        body = r.json()
+        assert body["result"] == [
+            {"id": 1, "name": "a"},
+            {"id": 2, "name": "b"},
+        ]
+
+    def test_optional_basemodel_returns_inner_or_none(self, http):
+        r_found = http.post("/api/mizan/call/", json={"fn": "find_item", "args": {"item_id": 5}})
+        assert r_found.status_code == 200
+        assert r_found.json()["result"] == {"id": 5, "name": "found"}
+
+        r_missing = http.post("/api/mizan/call/", json={"fn": "find_item", "args": {"item_id": 0}})
+        assert r_missing.status_code == 200
+        assert r_missing.json()["result"] is None
+
+
+# ─── Merge protocol ─────────────────────────────────────────────────────────
+
+
+class AsyncHandlerTests:
+    """`async def` handlers dispatch on the loop via view.acall."""
+
+    def test_async_handler_returns_awaited_result(self, http):
+        r = http.post("/api/mizan/call/", json={"fn": "async_echo", "args": {"text": "hello"}})
+        assert r.status_code == 200
+        assert r.json()["result"] == {"message": "async: hello"}
+
+
+class MergeTests:
+    """@client(merge=...) emits a `merge` field in the response so the kernel can splice without refetch."""
+
+    def test_merge_target_emits_merge_entry(self, http):
+        r = http.post(
+            "/api/mizan/call/",
+            json={"fn": "set_item_name", "args": {"id": 42, "name": "renamed"}},
+        )
+        assert r.status_code == 200
+        body = r.json()
+        # Server resolves slot — items_list returns list[ItemOutput], mutation returns ItemOutput
+        assert body["merge"] == [
+            {"context": "items", "slot": "items_list", "value": {"id": 42, "name": "renamed"}}
+        ]
+        # invalidate stays empty when only merge is declared
+        assert body["invalidate"] == []