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>

components/studio2/Details.vue

@@ -17,6 +17,7 @@ const rows=computed(()=>{const m=props.metadata||{},s=m.settings||{};const time=
  {label:'Identity still',value:m.identityStillId||'None'},{label:'Face lock',value:m.lockFace!==false?'On':'Off'},{label:'Outfit lock',value:m.lockOutfit!==false?'On':'Off'},
  {label:'Aspect',value:s.aspect||'auto'},{label:'FPS',value:s.fps||24},{label:'Hide input',value:m.hideInput?'On':'Off'},
  {label:'Start frame',value:m.startFrameSource?JSON.stringify(m.startFrameSource):'None'},
+ {label:'Handoff source',value:m.handoffSource || 'None'},{label:'Handoff fitted (resampled)',value:m.fitted==null?'Not recorded':m.fitted?'Yes':'No'},
  {label:'End still',value:m.endStillId||'None'},{label:'Guide stills',value:m.guides?.length?JSON.stringify(m.guides):'None'},
  {label:'Compiled prompt',value:m.compiledPrompt},{label:'Queued',value:time(m.queuedAt)},{label:'Started',value:time(m.startedAt)},{label:'Saved',value:time(m.savedAt)},
  {label:'GPU seconds',value:m.gpuSeconds},{label:'Wall seconds',value:m.wallTime},{label:'Comfy prompt ID',value:m.promptId},{label:'Host cleanup',value:m.purgeResult}