Teach both engines the sticker wire

A sticker is a content envelope carrying an id, never an image: sending the
picture would cost an upload and a download of bytes both ends already ship
with, on every sticker. This is about sixty bytes.

Recognised before the text path on both sides, because a sticker is renderable
but is NOT text - it draws as a glyph with no bubble, and falling through would
render it as an empty message. An unknown pack or id has to degrade to the
pack's fallback rather than break, which is what the envelope format exists for.

The pack is one JSON manifest read by both clients, so there is a single source
of truth for what exists. docs/STICKERS.md says plainly what the first cut is:
the glyphs are Unicode, because real sticker art is a design job, not a code
one. The protocol, picker, rendering, fallback and tests are real and finished;
swapping in artwork is a change to the manifest plus an image loader, not to
any of that. It is not a claim to have drawn a sticker set.

Emoji are deliberately NOT this. They go into the text of an ordinary message
and are indistinguishable from any other character once sent - no new kind, no
new field, nothing for the receiver to understand. That picker is a keyboard
convenience, not a protocol feature.

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

app/src/main/assets/stickers/flamenet.core.json

@@ -0,0 +1,31 @@
+{
+  "pack": "flamenet.core",
+  "name": "Flamenet",
+  "fallback": "🙂",
+  "stickers": [
+    { "id": "wave",     "glyph": "👋", "name": "Wave" },
+    { "id": "yes",      "glyph": "👍", "name": "Yes" },
+    { "id": "no",       "glyph": "👎", "name": "No" },
+    { "id": "laugh",    "glyph": "😂", "name": "Laughing" },
+    { "id": "love",     "glyph": "❤️", "name": "Love" },
+    { "id": "party",    "glyph": "🎉", "name": "Party" },
+    { "id": "think",    "glyph": "🤔", "name": "Thinking" },
+    { "id": "cry",      "glyph": "😭", "name": "Crying" },
+    { "id": "fire",     "glyph": "🔥", "name": "Fire" },
+    { "id": "clap",     "glyph": "👏", "name": "Applause" },
+    { "id": "sleep",    "glyph": "😴", "name": "Sleeping" },
+    { "id": "eyes",     "glyph": "👀", "name": "Looking" },
+    { "id": "ok",       "glyph": "👌", "name": "OK" },
+    { "id": "pray",     "glyph": "🙏", "name": "Please" },
+    { "id": "strong",   "glyph": "💪", "name": "Strong" },
+    { "id": "coffee",   "glyph": "☕", "name": "Coffee" },
+    { "id": "rocket",   "glyph": "🚀", "name": "Rocket" },
+    { "id": "bug",      "glyph": "🐛", "name": "Bug" },
+    { "id": "check",    "glyph": "✅", "name": "Done" },
+    { "id": "warning",  "glyph": "⚠️", "name": "Careful" },
+    { "id": "wave-bye", "glyph": "🫡", "name": "Salute" },
+    { "id": "shrug",    "glyph": "🤷", "name": "Shrug" },
+    { "id": "star",     "glyph": "⭐", "name": "Star" },
+    { "id": "cake",     "glyph": "🎂", "name": "Cake" }
+  ]
+}