Address review: split semantics, status note, DM size constraint, vacuous parity test

- Parsing Rule 1 now says 'at most 5 parts' and spells out the
  Swift maxSplits:4 vs Rust splitn(5) off-by-one, which would otherwise
  make a splitn(4) implementor reject every valid reference.
- Overview gains an explicit Status note: nothing parses, routes, fetches,
  or renders sticker references yet (reviewer ask on #1544).
- Document the 255-byte DM content constraint for maximal-length
  references, pointing at #784 (Codex P2 thread).
- parseAcceptsSonarFfiEncodedContent now #requires the parse result:
  #expect(ref == makeRef(...)) could pass vacuously as nil == nil.

Flagged by the multi-model review panel.

bitchatTests/Protocols/StickerRefCodecTests.swift

@@ -51,10 +51,12 @@ struct StickerRefCodecTests {
         #expect(StickerRefCodec.encode(ref).hasPrefix("\u{1F}sticker\u{1F}"))
     }
 
-    @Test func parseAcceptsSonarFfiEncodedContent() {
+    @Test func parseAcceptsSonarFfiEncodedContent() throws {
         // Hand-built exactly as mesh_parse_sticker_content would see it.
         let wire = "\u{1F}sticker\u{1F}30031:\(pubkey):pack\u{1F}wave\u{1F}\(sha256)"
-        let ref = StickerRefCodec.parse(wire)
+        // #require (not #expect against makeRef): if a regression made parse
+        // return nil here, `nil == nil` would let the test pass vacuously.
+        let ref = try #require(StickerRefCodec.parse(wire))
         #expect(ref == makeRef(coordinate: "30031:\(pubkey):pack"))
     }
 

docs/SONAR-STICKERS.md

@@ -15,6 +15,19 @@ specifies only the bitchat wire format and client behavior. The reference
 implementation is byte-identical to sonar-ffi's `mesh_sticker_content` /
 `mesh_parse_sticker_content`.
 
+> **Status:** this lands in slices. The wire codec below is merged first as
+> dead-on-arrival infrastructure — **nothing in the client parses, routes,
+> fetches, or renders sticker references yet.** Parsing, consent-gated
+> rendering, and opt-in sync arrive in the follow-up PRs (see the split of
+> #1517). Until then, sticker content is ordinary text to every client.
+
+> **DM size constraint:** a maximally long reference (80-char identifier +
+> 64-char shortcode) is ~290 bytes, which exceeds the current 255-byte
+> private-message content limit (`PrivateMessagePacket`). Senders SHOULD keep
+> the encoded reference within 255 bytes for DMs until that limit is raised
+> (tracked in #784); oversized references remain valid on public mesh and
+> geohash channels.
+
 ## Wire Format
 
 ### Content Prefix
@@ -46,8 +59,11 @@ Field order and count are fixed:
 
 Parsers MUST:
 
-1. Split the content on `0x1F` with **at most 4 splits**, preserving empty
-   subsequences.
+1. Split the content on `0x1F` into **at most 5 parts**, preserving empty
+   subsequences. Beware off-by-one semantics across languages: this is
+   Swift's `split(separator:maxSplits:)` with `maxSplits: 4` (4 splits → up
+   to 5 parts), but Rust's `splitn(5, ...)` — `splitn(4, ...)` yields at
+   most 4 parts and would reject every valid reference.
 2. Accept only if the split yields **exactly 5 parts**, `parts[0]` is empty,
    and `parts[1] == "sticker"`.
 3. Validate fields 2–4 against the table above; any violation MUST cause the