Eyevinn / mp4ff / 22222966201

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feat: add streaming/incremental processing for fragmented MP4 files.

Add Copy method for StypBox and FtypBox.

Implements support for processing fragmented MP4 files incrementally without
loading entire files into memory. This is essential for handling large media
files, live streaming scenarios, and network streams where fragments arrive
over time.

Key components:

1. StreamFile API (stream.go)
   - DecodeStream/InitDecodeStream: Reads init segment (ftyp, moov) and stops
   - ProcessFragments: Iteratively processes fragments with callbacks
   - Sliding window memory management with configurable retention
   - Fragment lifecycle callbacks for processing and cleanup control

2. BoxSeekReader (boxseekreader.go)
   - Emulates seeking on non-seekable streams using buffered reading
   - Auto-growing buffer sized for current needs
   - Peek operations for looking ahead without consuming data

3. SampleAccessor interface
   - GetSample: Fetch individual samples (1-based indexing)
   - GetSampleRange: Fetch ranges of samples efficiently
   - GetSamples: Fetch all samples for a track

Buffer management:
- ReadFullBox returns a slice view of the buffer (no copy for performance)
- Callers must use data immediately before next operation
- ResetBuffer() explicitly clears buffer after parsing is complete
- Buffer clearing happens after box parsing, not during ReadFullBox
- For mdat seeks, ResetBuffer() called after seek to avoid buffer corruption

Testing:
- Comprehensive test suite using external black-box testing (mp4_test package)
- Tests cover streaming, callbacks, sliding windows, retention, sample access
- All tests use public API only per project guidelines

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