Simplify segmentation spec

[igor-sirotin]

The segmentation spec was merged with several review comments deferred.
This issue tracks the remaining work on this spec.

Main task is to simplify. The protocol is simple, but the spec is very long.

• Focus on wire format
• Remove duplications
• Remove fluff

Follow-ups

The summary below is AI-generated, just to gather them in one place. Refer to the source comments.

• Enforce that the entire SegmentMessageProto (not just payload) fits within segmentSize. Current spec defines segmentSize as the payload chunk size before protobuf serialization. comment.
• Collapse data_segment_index and parity_segment_index into a single index field — current asymmetry is unnecessary. comment, comment.
• Re-cast the Receiving section as wireformat-focused normative rules rather than implementation prose (define abstract types, then concrete types, then validity rules). comment.
• Mark each SegmentMessageProto field as required/optional via a table or tagging instead of prose. comment.
• Audit key words usage against RFC 2119, comment.
• Expand mplementation Suggestions with concrete guidance for implementers (cache eviction strategies, circular buffers, memory-footprint handling) instead of just listing requirements. comment.
• Add guidance on when parity segments are worthwhile vs. when they're wasted bandwidth. comment.