Improve BLE mesh flooding: last-hop suppression, K-of-N fanout, and backpressure (#517)

* Improve BLE mesh relay and flooding

Add last-hop suppression using ingress-link tracking to prevent echo. Implement deterministic always-relay for handshakes and directed encrypted/fragments; widen jitter for broadcasts; keep TTL cap only for broadcast. Add deterministic K-of-N broadcast fanout to reduce amplification in dense topologies. Introduce backpressure-aware writes using canSendWriteWithoutResponse with per-peripheral queues and draining on peripheralIsReady. Minor helpers for messageID, deterministic selection, and maintenance cleanup.

* Tests: stabilize FragmentationTests and InputValidatorTests

Make _test_handlePacket mark synthetic peers verified/connected with normalized senderID to avoid drops in public-message reassembly tests. Tighten validatePeerID to reject non-hex strings when length equals 16 or 64; allow internal IDs only for other lengths. All iOS simulator tests pass locally.

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Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/Utils/InputValidator.swift

@@ -18,14 +18,18 @@ struct InputValidator {
     
     /// Validates a peer ID from any source (short 16-hex, full 64-hex, or internal alnum/-/_ up to 64)
     static func validatePeerID(_ peerID: String) -> Bool {
-        // Accept short routing IDs (16-hex)
+        // Accept short routing IDs (exact 16-hex)
         if PeerIDResolver.isShortID(peerID) { return true }
-        // Accept full Noise key hex (64-hex)
+        // If length equals short-hex length but isn't valid hex, reject
+        if peerID.count == Limits.hexPeerIDLength { return false }
+        // Accept full Noise key hex (exact 64-hex)
         if PeerIDResolver.isNoiseKeyHex(peerID) { return true }
-        // Internal format: alphanumeric + dash/underscore up to 64
+        // If length equals full key length but isn't valid hex, reject
+        if peerID.count == Limits.maxPeerIDLength { return false }
+        // Internal format: alphanumeric + dash/underscore up to 63 (not 16 or 64)
         let validCharset = CharacterSet.alphanumerics.union(CharacterSet(charactersIn: "-_"))
         return !peerID.isEmpty &&
-               peerID.count <= Limits.maxPeerIDLength &&
+               peerID.count < Limits.maxPeerIDLength &&
                peerID.rangeOfCharacter(from: validCharset.inverted) == nil
     }