Bridge dedup: content-derived stable mesh message ID (#1419)

* Bridge dedup keys on a content-derived stable mesh message ID

Public mesh messages carry no message ID on the BLE wire, so every
non-origin device minted a fresh UUID and the bridge's m-tag dedup only
matched on the origin device: duplicate bridged rows, misattributed
"across the bridge" counts, redundant downlink rebroadcasts, and an
m-tag spoof vector (an attacker could claim a victim's message ID).

Every device now derives the same stable ID from the signed wire fields
(sender ID + ms timestamp + trimmed content, SHA256/32 hex) via the new
MeshMessageIdentity — zero BLE wire change. The bridge event's m tag
carries the origin coordinates ["m", senderIDHex, timestampMs] and
receivers recompute the key from those plus the event's own content
instead of trusting a claimed ID; old-format/absent tags fall back to
the event ID as before.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Fix mixed-version message loss: m tag leads with the derived stable ID

The previous layout (["m", senderIDHex, timestampMs]) broke v1.7.0
receivers: their parser takes m[1] unconditionally as the timeline
dedup key whenever the tag has >= 2 elements, so every bridged message
from a new-version sender keyed on the CONSTANT sender hex and
inject-dedup dropped all but the first. The tag is now
["m", <derived stable ID>, senderIDHex, timestampMs]: old parsers get a
per-message-unique m[1] (exactly today's semantics), while the new
parser recomputes the ID from elements 2-3 plus the event's own content
and never trusts element 1, keeping the recompute-don't-trust property.

Also:
- Soften the overstated security claim in MeshMessageIdentity and the
  BridgeService classify comment: forging a chosen ID onto different
  content is infeasible, but all three hash inputs are cleartext on the
  radio, so identical-content front-running by a radio-local attacker
  remains possible (no worse than the unbridged mesh).
- Fix the stale archivedEchoKeys rationale: re-synced copies of others'
  messages now carry the derived stable ID (insert-by-ID catches them);
  the content key remains for echo--prefixed archive rows + self echoes.
- Tests: old-parser semantics on the new tag (m[1] per-message-unique
  and equal to the derived ID) and a forged-m[1] event that cannot
  pre-poison a genuine message's dedup slot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

bitchat/Nostr/NostrProtocol.swift

@@ -264,22 +264,33 @@ struct NostrProtocol {
 
     /// Create a mesh-bridge public message (kind 20000) for a geohash-cell
     /// rendezvous. The distinct `r` tag keeps bridge traffic out of geohash
-    /// channel subscriptions (which filter on `#g`); `m` carries the original
-    /// mesh message ID so receivers dedup the bridged copy against the radio
-    /// copy by timeline ID.
+    /// channel subscriptions (which filter on `#g`); `m` is
+    /// `[stable ID, mesh sender ID, wire timestamp in ms]`. Element 1 is the
+    /// content-stable mesh message ID (`MeshMessageIdentity`) for v1.7.0
+    /// parsers, which key their dedup on `m[1]` unconditionally and need it
+    /// per-message-unique; current parsers ignore it and recompute the ID
+    /// from the origin coordinates (elements 2-3) plus the event's own
+    /// content to dedup the bridged copy against the radio copy by
+    /// timeline ID.
     static func createBridgeMeshEvent(
         content: String,
         cell: String,
         senderIdentity: NostrIdentity,
         nickname: String? = nil,
-        meshMessageID: String? = nil
+        meshSenderID: String? = nil,
+        meshTimestampMs: UInt64? = nil
     ) throws -> NostrEvent {
         var tags = [["r", cell]]
         if let nickname = nickname?.trimmedOrNilIfEmpty {
             tags.append(["n", nickname])
         }
-        if let meshMessageID = meshMessageID?.trimmedOrNilIfEmpty {
-            tags.append(["m", meshMessageID])
+        if let meshSenderID = meshSenderID?.trimmedOrNilIfEmpty, let meshTimestampMs {
+            let stableID = MeshMessageIdentity.stableID(
+                senderIDHex: meshSenderID,
+                timestampMs: meshTimestampMs,
+                content: content
+            )
+            tags.append(["m", stableID, meshSenderID, String(meshTimestampMs)])
         }
         let event = NostrEvent(
             pubkey: senderIdentity.publicKeyHex,