phase 5: fix snapshot looking lower-quality than stream

User reported "snapshot is still lower qual than the stream" even at
jpegQuality=1. Two compounding causes in the sharp transform path:

1. Quality mapping topped out at 97. Previous formula was
   Math.max(50, 100 - v.jpegQuality * 3); at jpegQuality=1 that
   produced quality=97 — versus the stream's ffmpeg mjpeg -q:v 1
   which lands at sharp-equivalent ~99-100. Visible compression
   artifacts on flat regions accounted for ~half the perceived
   quality gap. New formula Math.min(100, 102 - v.jpegQuality * 2)
   yields:
     jpegQuality=1  → 100   (was 97)
     jpegQuality=2  →  98   (was 94)
     jpegQuality=5  →  92   (was 85)
     jpegQuality=10 →  82   (was 70)
     jpegQuality=31 →  40   (was 50, clamped)

2. Chroma subsampling was sharp's default 4:2:0 — half-rate chroma
   in U+V planes. On colored edges (UI overlays, sharp camera
   subjects against backgrounds, text) this smears and is the
   dominant visible artifact at panel-native 800x480. ffmpeg
   mjpeg's default behavior at -q:v 1 is closer to 4:2:2 or no
   subsampling. Force 4:4:4 when jpegQuality <= 2 (the "I want max
   quality" regime); keep 4:2:0 for jpegQuality >= 3 since the
   point at that quality is smaller files.

Plus mozjpeg: true on the encoder. Same quality value, slightly
tighter file size — modest but free.

The Lanczos kernel was already lanczos3 — confirmed, no change.

Expected outcome: snapshot at jpegQuality=1 is visually
indistinguishable from a stream frame at the same scene.

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