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>

examples/fastapi-react-site/backend/main.py

@@ -142,6 +142,58 @@ def current_user(request) -> UserOutput:
     )
 
 
+# ─── Merge protocol fixtures ────────────────────────────────────────────────
+
+
+class MorphGroupMeta(BaseModel):
+    """Group summary — narrower shape than MorphLayer. Listed alongside
+    morph_layers so the server's slot resolver has to discriminate by
+    return-type rather than by bundle order."""
+    id: int
+    label: str
+    count: int
+
+
+class MorphLayer(BaseModel):
+    id: int
+    group_id: int
+    label: str
+    value: float
+
+
+_morph_groups: list[MorphGroupMeta] = [
+    MorphGroupMeta(id=1, label="face", count=2),
+]
+
+
+_morph_layers: list[MorphLayer] = [
+    MorphLayer(id=1, group_id=1, label="brow", value=0.0),
+    MorphLayer(id=2, group_id=1, label="jaw", value=0.0),
+]
+
+
+@client(context="morphs")
+def morph_groups(request) -> list[MorphGroupMeta]:
+    """Summary-shape slot — server must route MorphLayer mutations away from here."""
+    return list(_morph_groups)
+
+
+@client(context="morphs")
+def morph_layers(request) -> list[MorphLayer]:
+    """Detailed-shape slot — server routes MorphLayer mutations here."""
+    return list(_morph_layers)
+
+
+@client(merge="morphs")
+def set_morph_value(request, id: int, value: float) -> MorphLayer:
+    """Mutation that returns the changed row; kernel splices into morph_layers."""
+    for layer in _morph_layers:
+        if layer.id == id:
+            layer.value = value
+            return layer
+    raise ValueError(f"unknown morph layer id={id}")
+
+
 # ─── Registration ───────────────────────────────────────────────────────────
 
 
@@ -156,6 +208,9 @@ register(not_implemented_fn, "not_implemented_fn")
 register(buggy_fn, "buggy_fn")
 register(permission_check_fn, "permission_check_fn")
 register(current_user, "current_user")
+register(morph_groups, "morph_groups")
+register(morph_layers, "morph_layers")
+register(set_morph_value, "set_morph_value")
 
 
 # ─── App ────────────────────────────────────────────────────────────────────