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/pyproject.toml

@@ -20,3 +20,8 @@ pythonpath = ["."]
 testpaths = ["."]
 python_classes = ["*Tests", "*Test", "Test*"]
 python_functions = ["test_*"]
+
+[dependency-groups]
+dev = [
+    "uvicorn>=0.47.0",
+]