Precise edits

To change a world, you do not rewrite the recipe. You send an edit transaction. It names the revision it starts from and lists the changes:

Sunward Grove before the edit — `coherent-kit-edit.json`: one transaction changes a colour, a tree top and a spin. Nothing else changes.

Sunward Grove after the edit — `coherent-kit-edit.json`: one transaction changes a colour, a tree top and a spin. Nothing else changes.

{"base_revision": 2, "edits": [
  {"part": "crown-top", "set": {"scale": [1.1, 0.65, 0.84]}},
  {"material": "gold", "set": {"emission": 0.8}},
  {"rhythm": "breeze", "set": {"period": 10}},
  {"add_part": {"id": "seed", "parent": "crown-top", "material": "gold",
    "geometry": {"kind": "ico_sphere", "radius": 0.05, "subdivisions": 1}}},
  {"remove_part": "old-branch"}
]}

Each edit changes exactly one part, material or rhythm.
Setting a rhythm name that does not exist yet adds it. remove_rhythm removes one.
Nested values merge. Changing a geometry kind replaces the old shape.
Removing a part keeps its children. Removing a rhythm that parts still follow fails.
A transaction holds 1–64 edits. Either all of them apply or none do. Blank checks the whole result, adds one to revision, and rejects a transaction whose base_revision is out of date.