Stop resampling the extend start frame every hop

upload() ran fitStill (ffmpeg scale+pad) on every image except hero,
including the start/handoff still. Four chained extends resampled the
anchor frame four times and destroyed identity even with the hero
reference and lock text intact.

- upload(): only fit when the still's aspect truly differs from the
  job canvas (needsFit); never fit hero or start regardless.
- Track handoffSource (anchor|extract) and whether the start frame was
  fitted; surface both in job metadata and Details.
- extendSelected(): re-binding an already-selected clip on task switch
  must not stomp the user's overridden locks/settings back to the
  clip's saved values (regression introduced by the last hotfix push).
- Add tests/studio2-extend-fit.test.mjs: two extends at identical
  width/height reach Comfy with the anchor PNG byte-identical, no
  ffmpeg fit.

Co-authored-by: Cursor <cursoragent@cursor.com>

shared/studio2/input.mjs

@@ -44,6 +44,9 @@ export function extendSelected(form, clip, xaigen = false) {
   const next = { ...form, mode: 'extend', engine: xaigen && form.engine === 'ltx' ? 'ltx' : 'minimax' }
   const target = clip?.id ? clip : null
   if (!target?.id) return next
+  // Already bound to this clip: switching task must not stomp the user's overrides
+  // (locks, CFG, etc.) by re-copying the clip's saved settings over them.
+  if (form.startClipId === target.id) return next
   try { return bindClip(next, target, xaigen) }
   catch { return { ...next, startClipId: target.id } }
 }