Delivered thumbnails drift out of sync after trimming

[johan-bell]

Reported, not yet investigated. After a trimmed encode, the scrub thumbnails no
longer match the frames they preview — the sprite cue shows a moment visibly away
from where the playhead lands. Observed on the multi-stream test source whose
streams start apart (alignment offset ~1.06 s).

The suspects, unverified but familiar. ThumbnailService.packForDelivery maps
source-timeline thumbs onto the output timeline, and makes two assumptions the
waveform sidecar was just cured of (item 46):

1. outDuration sums (outSec - inSec) unclamped, but buildConcatFile writes
   inpoint = max(inSec, alignmentOffset) — a trim range starting inside the head
   region contributes less than its face value, so every cue after it sits late by
   the difference. The waveform fix's Defect B was this same reduce; the corrected
   form is in encode.service.ts's sidecar block.
2. The non-trim path uses sourceDuration and no offset at all, while the
   encoded output starts at source t = alignmentOffset and is that much shorter —
   so on a misaligned source the delivered storyboard should be wrong even without
   a trim, cues early by ~the offset throughout. EncodeResult.alignmentOffset
   exists now (added for the waveform); packForDelivery never receives it.
3. selectThumbsForOutput picks source thumbs per output position via the ranges —
   whatever mapping it applies inherits both errors above; check it against the
   clamped in-points rather than the raw trim ranges.

Where. api/src/encode/services/thumbnail.service.ts (packForDelivery,
selectThumbsForOutput, orderedRanges), threaded from encode.service.ts where
encodeResult.alignmentOffset is already in scope. The pre-encode source storyboard
(trim UI filmstrip) draws on the source timeline and should be unaffected — verify
rather than assume.

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Migrated from Todo.md item 48, which is removed as the repository is prepared to go public.