Function

isPositionResumable

isPositionResumable(
positionSeconds: number | null,
durationSeconds: number,
): positionSeconds is number
Returns true when the saved positionSeconds is meaningful enough to resume from (not noise, not "basically finished"). Both bounds must hold.
Parameters
- positionSeconds: number | null
  The stored position, or null when nothing is saved
- durationSeconds: number
  The lesson's length; <= 0 is treated as unknown and never resumable
Returns positionSeconds is number
Whether the resume dialog should be offered for this position
Remarks
This is a presentation policy, not a domain invariant: it decides when it is worth offering a resume, and deliberately lives outside the hexagon. Range and finiteness of a persisted position are the PlaybackPosition value object's job.

It lives apart from any component so the player can gate on it without importing modal code — see openspec/changes/resume-dialog-shadcn-primitive/design.md §D4.
Example
```
isPositionResumable(180, 600); // true  — three minutes into a ten-minute lesson
isPositionResumable(5, 600);   // false — barely started
isPositionResumable(595, 600); // false — effectively finished
```
- Defined in lib/playback-resume-thresholds/playback-resume-thresholds.ts:44