refactor: remove DATA_RETRANSMISSION packet type and document announce-back

Since protocol v0.03 is already a breaking change, simplify the wire
protocol by removing the DATA_RETRANSMISSION type. The receiver now
checks the packet deduplicator for ALL incoming DATA packets. If the
packet group was already fully processed, an ACK is sent and the
fragment is discarded. This provides the same behavior with one fewer
packet type.

Packet type numbering is now:
  ANNOUNCE=1, LEAVE=2, DATA=3, ACK=4, NACK=5

Also document the announce-back behavior: when a node hears an announce
from an unknown peer, it immediately sends its own announce via unicast
to that peer. This gives instant bidirectional connection without
waiting for the next announce cycle.

Author: TrentHouliston

docs/explanation/nuclearnet.md

@@ -113,8 +113,9 @@ sequenceDiagram
     Note over A: New peer heard!
     A->>A: Add B to peer list
     A->>A: Fire NetworkJoin event
+    A->>B: AnnouncePacket (unicast reply)
 
-    Note over B: Hears A's announce
+    Note over B: Immediate connection
     B->>B: Add A to peer list
     B->>B: Fire NetworkJoin event
 
@@ -128,6 +129,11 @@ sequenceDiagram
     A->>A: Fire NetworkLeave event
 ```
 
+When a node hears an announce from an unknown peer,
+it immediately sends its own announce back via unicast directly to that peer.
+This eliminates the need to wait for the next periodic announce cycle —
+new peers connect within a single round trip rather than waiting up to one full announce interval.
+
 ### Announce address options
 
 The announce address can be:
@@ -180,14 +186,13 @@ A received packet is only accepted if the magic bytes, version, and type field a
 
 ### Packet types
 
-| Type                | Value | Purpose                                |
-| ------------------- | ----- | -------------------------------------- |
-| ANNOUNCE            | 1     | Periodic discovery broadcast           |
-| LEAVE               | 2     | Graceful departure notification        |
-| DATA                | 3     | Normal data payload                    |
-| DATA_RETRANSMISSION | 4     | Retransmitted data fragment            |
-| ACK                 | 5     | Acknowledgment of received fragments   |
-| NACK                | 6     | Request for specific missing fragments |
+| Type     | Value | Purpose                                |
+| -------- | ----- | -------------------------------------- |
+| ANNOUNCE | 1     | Periodic discovery broadcast           |
+| LEAVE    | 2     | Graceful departure notification        |
+| DATA     | 3     | Data payload (original or retransmission) |
+| ACK      | 4     | Acknowledgment of received fragments   |
+| NACK     | 5     | Request for specific missing fragments |
 
 ### Announce packet
 
@@ -324,7 +329,7 @@ sequenceDiagram
 
     Note over S: Sees fragment 1 not ACKed
     Note over S: Wait RTO timeout...
-    S->>R: DataRetransmission (id=42, no=1)
+    S->>R: DataPacket (id=42, no=1, retransmit)
 
     R->>S: ACK (id=42, bitset=[1,1,1])
     Note over R: All fragments received, deliver message

src/nuclearnet/NUClearNet.cpp

@@ -225,7 +225,6 @@ namespace network {
         for (const auto& req : retransmissions) {
             // Build header on stack, scatter-write header + fragment data
             DataPacket header{};
-            header.type         = DATA_RETRANSMISSION;
             header.packet_id    = req.packet_id;
             header.packet_no    = req.packet_no;
             header.packet_count = req.packet_count;
@@ -429,8 +428,7 @@ namespace network {
                 discovery->process_leave(source);
             } break;
 
-            case DATA:
-            case DATA_RETRANSMISSION: {
+            case DATA: {
                 if (length < sizeof(DataPacket)) {
                     return;
                 }
@@ -439,9 +437,8 @@ namespace network {
 
                 // Check for duplicates (at the packet group level)
                 auto& dedup = deduplicators[source];
-                bool is_retransmission = (header->type == DATA_RETRANSMISSION);
 
-                if (is_retransmission && dedup.is_duplicate(pkt->packet_id)) {
+                if (dedup.is_duplicate(pkt->packet_id)) {
                     // Already processed this packet group — send ACK if reliable
                     if ((pkt->flags & RELIABLE) != 0) {
                         std::vector<bool> all_received(pkt->packet_count, true);

src/nuclearnet/wire_protocol.hpp

@@ -41,12 +41,11 @@ namespace network {
 
     /// Packet type identifiers
     enum PacketType : uint8_t {
-        ANNOUNCE            = 1,
-        LEAVE               = 2,
-        DATA                = 3,
-        DATA_RETRANSMISSION = 4,
-        ACK                 = 5,
-        NACK                = 6,
+        ANNOUNCE = 1,
+        LEAVE    = 2,
+        DATA     = 3,
+        ACK      = 4,
+        NACK     = 5,
     };
 
     /// Data packet flags (bit field)
@@ -109,7 +108,7 @@ namespace network {
      * Data packet — carries message payload (possibly one fragment of a larger message).
      *
      * Wire layout (18+ bytes):
-     *   [0-4]   PacketHeader (type = DATA or DATA_RETRANSMISSION)
+     *   [0-4]   PacketHeader (type = DATA)
      *   [5-6]   packet_id (uint16_t) — unique identifier for this packet group
      *   [7-8]   packet_no (uint16_t) — fragment index within the group (0-based)
      *   [9-10]  packet_count (uint16_t) — total fragments in the group