Simplify segmentation spec

[igor-sirotin]

The segmentation spec was merged with several review comments deferred.
This issue tracks the remaining work on docs/messaging/standards/application/segmentation.md.

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.