MCP: add_effect is one undo step and leaves nothing applied when it fails (#175)

add_effect ran effect.apply and then one prop.set per `values` entry as
separate commands: each became its own undo step, and a bad value (an
unknown parameter id) returned an error but left the effect, and any
values set before it, on the layer, so a corrected retry landed at
effects/#2.

It now runs them as one engine.batch, named like effect.apply's own undo
step ("Apply Gaussian Blur"): one undo removes the effect with its values,
and a failing step rolls the whole call back. The reply is unchanged.

Test: add_effect_is_one_undo_step_and_all_or_nothing.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/agents.md

@@ -59,7 +59,7 @@ The server speaks JSON-RPC 2.0 over stdio, one message per line, and supports MC
 | `add_keyframe {layer, path, time+value \| keys:[...], interpolation?, comp?}` | Adds keys, then optionally applies linear/bezier/hold/easyEase. |
 | `list_effects {filter?}` | Effect ids, names, categories, GPU / 32-bpc support and parameters. |
 | `list_fonts {query?, rescan?}` | Font families text layers can use, bundled and installed, with their styles, origin and own-language name; `rescan` picks up fonts installed since launch. |
-| `add_effect {layer, effect, values?, comp?}` | Apply an effect and set its parameters in one call; returns the instance path (`effects/#n`) and its parameter paths. |
+| `add_effect {layer, effect, values?, comp?}` | Apply an effect and set its parameters in one call and one undo step (a failing value leaves nothing applied); returns the instance path (`effects/#n`) and its parameter paths. |
 | `render_frame {comp?, time?, max_side?, path?, inline?, transparent?}` | Returns a PNG image of a frame; `transparent: true` keeps the alpha (as an RGB + Alpha render writes it) instead of compositing over the comp background. |
 | `open_project {path \| demo \| new}` / `save_project {path?}` | Open and save files. |
 | `undo {steps?}` / `redo {steps?}` | History. |