fix(engine): flag single-keyframe videos as sparse instead of skipping them

[Abhishek-kumarsingh]

Summary

Fixes #3460. analyzeKeyframeIntervals returned isProblematic: false whenever a video had fewer than two keyframes, treating a single-GOP video the same as a still image. But a video with exactly one keyframe is the worst case for the failure mode the check exists to catch: every seek past 0 lands inside a single GOP spanning the whole file. A 10s single-GOP video went unreported while a video with a 5s max interval triggered the warning.

Fix

When exactly one keyframe is found, the effective interval is now the stream duration (fetched via the already-available extractMediaMetadata), not zero — matching the fix suggested in the issue. Still images and single-frame assets keep their current (non-problematic) behavior, since their duration is at or below the 2s threshold. Zero-keyframe handling is unchanged.

Test plan

• Added regression tests to packages/engine/src/utils/ffprobe.test.ts covering: a long-duration single keyframe (now flagged problematic), a short single keyframe (still not flagged), and zero keyframes (unchanged, not flagged) — confirmed the long-duration case failed before the fix
• Full @hyperframes/engine suite passes (1641/1641)
• oxlint and oxfmt --check clean on changed files

Note: the issue also raises a "side note" about whether the warning targets render's decode path at all, based on the reporter's own frame-accuracy measurements — that's a separate scoping question I've left alone; this PR only fixes the single-keyframe detection gap.