refactor: rename core-public -> engine-public; fold helpers in; drop shared

Tighten the package model around the determinism invariant: everything the
engine imports must itself be deterministic.

- Rename core-public -> engine-public (package, dir, `engine-public/*` alias,
  all 118 import sites). It is the deterministic public surface the engine
  depends on and that client/server also use.
- Move the pure formatting helpers (renderNumber/renderTroops) from shared
  into engine-public/format — the engine uses them and they're deterministic.
- Drop the now-empty shared package (and its alias). `shared` was intended for
  client/server-shared, engine-forbidden code; there is none yet, so it is
  removed and can be re-added when such code appears.

Final graph: engine-public (leaf) <- engine <- {client, server};
client/server also -> engine-public.

Verified: tsc clean; 1364 + 65 tests pass; prod build succeeds; engine-public
is a leaf; engine has no client/server imports. Lockfile regenerated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/client/src/WebGLFrameBuilder.ts

@@ -1,7 +1,7 @@
 import { Colord } from "colord";
 import { base64url } from "jose";
 import { assetUrl } from "engine/AssetUrls";
-import { decodePatternData } from "core-public/PatternDecoder";
+import { decodePatternData } from "engine-public/PatternDecoder";
 import { PlayerType } from "engine/game/Game";
 import { GameView } from "client/view";
 import { uploadFrameData } from "./render/frame/Upload";