Fix trun roundtrip correctness

[kixelated]

Fixes #186

Summary

• preserve the valid first_sample_flags layout and reject mixed per-sample field presence that cannot be encoded without changing inherited tfhd semantics
• decode version 0 composition offsets as unsigned and version 1 offsets as signed, widening TrunEntry::cts to Option<i64> so both ranges are representable
• select the compatible trun version when encoding composition offsets and reject incompatible or out-of-range values
• reject malformed trun boxes that set both first_sample_flags and sample_flags

Root cause and impact

The decoder and encoder did not use the trun version when reading composition offsets, and partially populated per-sample fields could be silently omitted or replaced with zero. This could lose key-frame flags or change values inherited from tfhd defaults during a decode/encode roundtrip.

The public TrunEntry::cts field changes from Option<i32> to Option<i64> to cover the complete v0 unsigned and v1 signed value ranges.

Validation

• cargo test (240 unit tests and 3 doc tests)
• cargo check --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• 11 targeted trun regression tests