Movie Editor: add 'carry i2v guide' scene source mode

A 4th scene source: the scene has no anchor of its own and continues from the
previous scene's i2v guide with overlap (chain-sampler carry behaviour). UI +
data model only; per-scene carry/reset in the sampler is the V2 work to follow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

movie_editor/backend/timeline.py

@@ -25,7 +25,7 @@ def _new_id() -> str:
 class SceneSource:
     """How a scene's latent is born. V1 ignores this (uniform chain); Phase 2 maps
     it onto EmptyLTXVLatent (empty/t2v) or LTXV Image to Video (image/i2v)."""
-    type: str = "empty"  # "empty" | "image" | "generated_frame"
+    type: str = "empty"  # "empty" | "image" | "generated_frame" | "carry"
     media_ref: Optional[str] = None              # asset id, for type == "image"
     frame_ref: Optional[dict[str, Any]] = None   # {scene_id, frame_idx}, for "generated_frame"
     target: Optional[str] = None                 # wire dest for the image: "port:<id>" | "node:<slotId>:<input>"

movie_editor/frontend/inspector.js

@@ -5,7 +5,12 @@
   const body = document.getElementById("inspector-body");
   const title = document.getElementById("inspector-title");
 
-  const SRC = [["empty", "Empty · text-to-video"], ["image", "Image · i2v anchor"], ["generated_frame", "From generated frame"]];
+  const SRC = [
+    ["empty", "Empty · text-to-video"],
+    ["image", "Image · i2v anchor"],
+    ["generated_frame", "From generated frame"],
+    ["carry", "Carry i2v guide · continue previous"],
+  ];
 
   function transitionSelect(value, onChange) {
     const sel = el("select");
@@ -157,6 +162,12 @@
 
     if ((scene.source?.type) === "image") renderImageSource(st, scene);
     if ((scene.source?.type) === "generated_frame") renderGeneratedFrameSource(st, scene);
+    if ((scene.source?.type) === "carry") {
+      const hint = el("div", "insp-block");
+      hint.append(el("div", "insp-hint",
+        "No start frame of its own — this scene continues from the previous scene's i2v guide and overlaps with it (chain-sampler carry behaviour). Use this for a continuous shot; use an Image / generated frame to hard-cut to a new anchor."));
+      body.append(hint);
+    }
 
     body.append(field("Transition to next scene", transitionSelect(scene.transition_to_next || "",
       (v) => S.patchScene(scene.id, { transition_to_next: v }))));

movie_editor/frontend/timeline.js

@@ -7,7 +7,7 @@
   const body = document.getElementById("timeline-body");
   const meta = document.getElementById("timeline-meta");
 
-  const SRC_ICON = { empty: "▦", image: "◐", generated_frame: "⛶" };
+  const SRC_ICON = { empty: "▦", image: "◐", generated_frame: "⛶", carry: "⇥" };
   const ZOOM_KEY = "funpack_tl_zoom";
   let pxPerSec = parseFloat(localStorage.getItem(ZOOM_KEY)) || 80;  // zoom
   let scrollLeft = 0;