docs: add large-message propagation flow and wire format draft

fetch/docs/docs/gossipsub-large-message-flow.md

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+# Gossipsub 1.4 Large Message Propagation
+
+## Overview
+
+This document describes the proposed large-message
+propagation mechanism for Gossipsub 1.4.
+
+Large payloads are segmented into smaller chunks
+before propagation through the gossip mesh.
+
+---
+
+# Segmentation Flow
+
+```text
+Publisher
+   │
+Large Message
+   │
+Segmentation Layer
+   │
+┌───────────────┐
+│ Segment 1     │
+│ Segment 2     │
+│ Segment 3     │
+└───────────────┘
+   │
+Gossipsub Mesh
+   │
+Receiving Peers
+   │
+Reassembly Layer
+   │
+Reconstructed Message
+```
+
+---
+
+# Segment Metadata
+
+Each segment contains:
+
+| Field | Description |
+|---|---|
+| message_id | Unique identifier for original message |
+| segment_index | Current segment number |
+| total_segments | Total number of segments |
+| checksum | Integrity verification |
+| payload | Segment data |
+
+---
+
+# Reconstruction
+
+Receiving peers reconstruct the message after all
+segments are received.
+
+Reconstruction includes:
+
+- ordered reassembly
+- integrity verification
+- duplicate filtering
+- missing segment recovery
+
+---
+
+# Security Considerations
+
+Potential concerns include:
+
+- malicious segment flooding
+- incomplete message spam
+- memory exhaustion attacks
+- invalid checksum propagation
+
+Integrity verification should occur before reassembly.