feat(image-list)!: replace fixed Start extras with growable value_N slots

Distributed Image List Start no longer has reference_image, text,
strength, seed or per-slot mode dropdowns. It now takes images, mask and
up to 10 generic value_N inputs (IMAGE, MASK, STRING, INT, FLOAT,
BOOLEAN) that grow via native V3 Autogrow, each with a matching output.

- One value is shared by every image; one per image pairs in order; any
  other count fails before dispatch. Only images is split into frames.
- Outputs are declared up front (image, mask, item_index, context,
  value_1..value_10). web/imageListStart.js hides unused value outputs,
  mirrors input types, and follows Autogrow compaction so downstream
  links keep their value.
- Image-list protocol bumped to v2; older workers fail preflight.

Breaking: saved workflows using the old Start inputs must recreate Start.

tests/test_dispatch_selection.py

@@ -168,7 +168,7 @@ class _FakeSession:
 def _valid_capability():
     return {
         "supported": True,
-        "protocol_version": 1,
+        "protocol_version": 2,
         "node_ids": ["DistributedImageListStart", "DistributedImageListEnd"],
     }
 
@@ -418,7 +418,7 @@ class DispatchSelectionTests(unittest.IsolatedAsyncioTestCase):
                 await dispatch.preflight_image_list_workers(workers, require_idle=True)
 
     async def test_image_list_preflight_fails_closed_for_malformed_capabilities(self):
-        malformed = {"supported": True, "protocol_version": 1, "node_ids": None}
+        malformed = {"supported": True, "protocol_version": 2, "node_ids": None}
         session = _FakeSession({("worker", "capabilities"): (200, malformed)})
 
         async def fake_session():
@@ -432,7 +432,7 @@ class DispatchSelectionTests(unittest.IsolatedAsyncioTestCase):
 
     async def test_image_list_preflight_fails_closed_for_incompatible_protocol(self):
         incompatible = _valid_capability()
-        incompatible["protocol_version"] = 2
+        incompatible["protocol_version"] = 1
         session = _FakeSession({("worker", "capabilities"): (200, incompatible)})
 
         async def fake_session():