maarcadetweet: initial commit

AT Protocol PDS + AppView + Tauri Desktop Client, 160-char post limit.

- PDS (Rust + axum + sqlx)
  - Auth: createAccount, createSession, refreshSession
  - Records: createRecord, deleteRecord (race-safe via SELECT FOR UPDATE)
  - Feed: feed.like.create, feed.repost.create
  - Sync: getRepo, getBlocks, getLatestCommit, getRecord (with MST proof), listRepos
  - Identity: resolveHandle
  - MST: spec-conformant (at-mst crate, 27 tests)
  - Repo: signed commits, TID counter (monotonic, 4096 wrap safe)

- AppView (Rust + axum + sqlx)
  - Jetstream consumer (WebSocket, exponential backoff, 38k+ events indexed)
  - REST API: timeline/home (graph-aware), profile, search, post (with thread hydration)
  - Handle-sync worker (did:plc + did:web)
  - JSONB embed storage + thread columns (migration 0003)
  - Like/repost counter cache (migration 0004)

- Tauri 2 + Svelte 5 Desktop Client
  - System tray (Show/Compose/Quit menu)
  - OS notifications (tauri-plugin-notification)
  - Auto-update (tauri-plugin-updater, placeholder endpoint)
  - Window-state (tauri-plugin-window-state)
  - 160-char compose with live counter
  - Image/Link embed rendering
  - LocalStorage-persisted like state
  - Timeline with poll (prepend new posts)
  - Custom TitleBar (transparent, no decorations)
  - Orange/IBM Plex Mono maarcade design

Tests: 231 Rust + 9 vitest = 240 passed.

lexicons/app/twi/post.json

@@ -0,0 +1,43 @@
+{
+  "lexicon": 1,
+  "id": "app.twi.post",
+  "defs": {
+    "main": {
+      "type": "record",
+      "key": "tid",
+      "record": {
+        "type": "object",
+        "required": ["text", "createdAt"],
+        "properties": {
+          "text": {
+            "type": "string",
+            "maxLength": 160,
+            "maxGraphemes": 160,
+            "description": "Post body. Capped at 160 characters (oldschool Twitter)."
+          },
+          "createdAt": {
+            "type": "datetime",
+            "description": "Client-supplied ISO-8601 timestamp; PDS may rewrite."
+          },
+          "reply": {
+            "type": "ref",
+            "ref": "app.bsky.feed.post#replyRef"
+          },
+          "embed": {
+            "type": "union",
+            "refs": [
+              "app.bsky.embed.images",
+              "app.bsky.embed.external",
+              "app.bsky.embed.record",
+              "app.bsky.embed.recordWithMedia"
+            ]
+          },
+          "langs": {
+            "type": "array",
+            "items": { "type": "string", "format": "language" }
+          }
+        }
+      }
+    }
+  }
+}