fix: multi-file timeline drop spacing

[miguel-heygen]

Problem

Studio could still produce a bad timeline layout when multiple media files were dropped onto the timeline at once.

The files uploaded correctly, but the placement logic treated the group too loosely:

• MP4 / MOV / MP3 files could be inserted with the same drop start instead of being spaced by their actual media durations
• dropping onto a row that already contained a long clip could leave the first new media clip overlapping existing content on that same track
• if duration metadata was unavailable during group placement, the helper could treat a clip as zero-length even though insertion would later write a positive fallback duration
• the saved HTML could therefore look valid at a glance while still creating track collisions during preview / static guard checks

What this fixes

Multi-file timeline placement

• resolves each uploaded file's media duration before placing the dropped group
• places the group sequentially from the drop point instead of assigning every file the same start time
• falls back to a finite placement duration when duration metadata is unavailable, keeping placement math aligned with the insert fallback path
• reuses the resolved duration when writing the inserted clip HTML, so the saved data-duration matches the placement math when metadata is available
• keeps the whole dropped group on one clear row so the assets read as a sequence instead of scattering across rows

Occupied-row handling

• keeps the requested drop track when the time range is clear
• if the requested row is already occupied at that time range, moves the whole group to a new clear track below the current timeline rows
• covers the default { start: 0, track: 0 } placement path when track 0 is already occupied
• preserves the existing track / z-index update path so older clips keep rendering predictably after the new clips are inserted

Media coverage

• keeps MP4 and MOV handled as video assets
• keeps MP3 handled as audio
• adds regression coverage for MOV / MP3 detection, occupied-row fallback, empty drops, clear high-track drops, and mixed positive / missing durations

Root cause

The multi-file drop helper only knew about the number of uploaded files. It did not know the duration of each uploaded media asset, so it could not build a non-overlapping sequence from the drop point.

The direct file-drop handler also passed placement into each insert independently. That meant the drop could use the same start time repeatedly, and it could target a row that was already occupied by an existing clip.

A smaller edge case existed when a duration resolved to 0 or was otherwise unavailable: placement math did not advance nextStart, but the insert path could still write a positive fallback duration later. The helper now uses a finite fallback for placement too.

Behavior

• dropping a single file still places that file at the dropped time / track when the row is clear
• dropping multiple files now creates a sequential group from the dropped time
• if a file's duration is unavailable during placement, the next file is still spaced forward by a finite fallback
• if the dropped row is occupied, the group moves together to the next clear track
• video and audio durations come from metadata when available, with the existing fallback duration still used when metadata cannot be read quickly
• inserted clips are still persisted into source HTML and the preview refreshes after the drop

Verification

Local checks

• bun run --filter @hyperframes/studio test -- src/utils/timelineAssetDrop.test.ts -> 12 tests pass
• bun run --filter @hyperframes/studio test -> 221 tests pass
• bun run --filter @hyperframes/studio typecheck
• bunx oxlint packages/studio/src/utils/timelineAssetDrop.ts packages/studio/src/utils/timelineAssetDrop.test.ts
• bunx oxfmt --check packages/studio/src/utils/timelineAssetDrop.ts packages/studio/src/utils/timelineAssetDrop.test.ts
• bun run --filter @hyperframes/studio build
• git diff --check
• pre-commit hook: lint / format / typecheck

Browser / live verification

Verified against a live local Studio fixture with real media files:

• dropped a group containing MP4, MOV, and MP3 files onto the timeline
• confirmed the saved HTML sequenced them at 2.18s, 3.38s, and 4.98s
• confirmed the existing background clip stayed on track 0 while the new media group moved together to clear track 1
• confirmed agent-browser errors was clean after the drop
• captured an agent-browser recording pass for the tested flow

Notes

• the timeline-multi-drop-repro Studio project and generated MP4 / MOV / MP3 files used for browser verification are local-only and are not part of this PR
• this PR is intentionally scoped to multi-file drop placement; it does not add broader timeline editing modes or change drag/move behavior for existing clips

[jrusso1020]

Multi-file drop placement looks good after the b4ac293 follow-up. All edge cases I flagged are closed:

• Zero/missing-duration silent overlap → fixed via FALLBACK_TIMELINE_FILE_DROP_DURATION = 5. Placement math now always advances; the new "uses fallback spacing when a duration is unavailable" test pins the behavior.
• Default-track at-time-0 occupied → covered by the new "moves a default-track drop to a clear row" test.
• Empty drop set → covered.
• Clear high-track placement → covered.

Verified locally on b4ac293:

• bun test packages/studio/src/utils/timelineAssetDrop.test.ts → 12/12 pass
• bun run --filter @hyperframes/studio test → 221/221 pass
• Studio typecheck clean

Sequenced placement with same-track overlap detection is the right model for "user dropped these as a group." The clamp-to-positive on clip.start / clip.duration, and 2dp rounding match the rest of the timeline math. Effect dependency on [activeCompPath, handleTimelineAssetDrop, timelineElements, uploadProjectFiles] is correct for the new closure.

Low blast radius — Studio drag-drop only, no render-pipeline impact.

— Rames Jusso