sidx encode: reference_size bit 31 bleeds into reference_type; SAP fields silently truncated

[kixelated]

src/sidx.rs:104-107:

let reference_type_and_size: u32 = match reference.reference_type {
    true => 0x8000_0000 | reference.reference_size,
    false => reference.reference_size,
};

SegmentReference.reference_size is a public u32, but the wire field is 31 bits (ISO/IEC 14496-12 §8.16.3). With reference_type: false and a reference_size that has bit 31 set, the encoded stream has the reference_type bit set: decoding yields reference_type: true with the truncated size — decode(encode(x)) != x, and the output claims a segment-index reference instead of a media reference. Following the crate's own convention for reference_count a few lines up (sidx.rs:97-101), out-of-range values should produce an Error rather than corrupt adjacent bits.

Milder variants in the same loop: sap_type (& 0b111) and sap_delta_time (& 0x0FFF_FFFF) are silently truncated instead of erroring, so out-of-range values also round-trip unequal — though at least they can't corrupt neighboring fields.

(Version selection for earliest_presentation_time/first_offset, the starts_with_sap bit, and the capped preallocation were all checked and are correct.)

Found during an extensive automated correctness review (Claude Code); verified against the current main sources.