fix(ios): honor explicit bitrate in compressVideo

[thangdevalone]

Summary

Fixes #3 - the bitrate parameter in compressVideo was declared but completely unused on iOS.

Root Cause

iOS used AVAssetExportSession for all compression paths, which only supports Apple's built-in quality presets (Low/Medium/High/1080p/720p etc.) and does not accept custom bitrate values. The bitrate parameter was accepted in the function signature but never referenced in the function body.

Solution

Added a new AVAssetWriter + AVAssetReader pipeline (compressWithBitrate) that activates when:

• bitrate > 0 AND
• • targetSizeInMB is not set (<= 0)
    This mirrors how Android handles explicit bitrate via Transformer's VideoEncoderSettings.setBitrate().

Key implementation details:

• H.264 encoding with AVVideoAverageBitRateKey for precise bitrate control
• • Audio passthrough (no re-encode) unless muteAudio: true
• • maxWidth support via CIImage GPU-accelerated resize when scaling is needed
• • Rotation preserved via preferredTransform on the writer input
• • Anti-inflation fallback - reverts to original file if encoder produces larger output
• • Existing paths unchanged - quality preset and targetSizeInMB smart compress are not affected

Behavior matrix:

Parameters	Before	After
{ bitrate: 4000000 }	Ignored, uses medium preset	Encodes at 4Mbps
{ quality: "low" }	Low preset	Unchanged
{ targetSizeInMB: 5 }	Smart compress	Unchanged
{ bitrate: 4000000, muteAudio: true }	Bitrate ignored	4Mbps + no audio

Testing

Needs macOS build + device testing with the test cases above.