Migrate to ESM: viewer to Vite first, then engine to NodeNext

[coyotte508]

Follow-up to #328 (Lost Fleet merge). The repo should move to ESM throughout. Today:

• The engine is CommonJS (tsc → dist/, consumed by the BGS game-server via dynamic import(), which already supports ESM).
• The viewer builds with vue-cli 4 / webpack 4 / ts-loader 6, and compiles the engine's sources directly. This pipeline is also the root cause of the CJS/ESM default-import interop bugs that broke the published UMD bundle (worked around by vendoring assert / shuffle-seed in engine/src/utils/).

Constraint / sequencing

True engine ESM ("type": "module" + module: NodeNext) requires .js extensions on ~200 relative imports — which ts-loader 6 / webpack 4 cannot resolve when compiling engine sources for the viewer. So the migration has to happen in this order:

1. Viewer → Vite

• Replace vue-cli 4 / webpack 4 with Vite + @vitejs/plugin-vue2 (Vue 2.7 already in use; class components are supported).
• Keep the two build targets: dev app (dist/app) and the self-contained UMD/IIFE lib built from src/wrapper.ts (dist/package, gaiaViewer global, bootstrap-vue external, svg assets inlined).
• Replace mochapack with vitest (or keep mocha via vite-node). ~810 existing specs must keep passing.
• Bonus: kills the pinned TypeScript 3.9/4.9 skew and the tsconfig.build.json transpile-only workarounds.

2. Engine → ESM

• "type": "module", module/moduleResolution: NodeNext, .js extension rewrite on relative imports (mechanical).
• Mocha → ESM runner (mocha --loader=ts-node/esm or vitest), update prepublishOnly.
• Verify the BGS game-server loads dist/wrapper.js as ESM (it JSON-round-trips results already, and the entry is loaded with dynamic import()).
• Drop the remaining CJS-only dep quirks (seedrandom interop shim in src/utils/shuffle.ts can become a plain default import).

3. Cleanup enabled by the above

• Remove the interop-proof import shims and tsconfig.build.json special-casing.
• Consider excluding src/ai/ + src/fuzz/ from the published engine tarball (files), they are dev-only.
• Prune the now-moot statistics=charts preference from the BGS game doc (stats window is table-only).

Since engine 4.10+ we are allowed to break seed/replay compat if it ever helps (v3 beta has no games), but the vendored shuffle is byte-identical anyway, so nothing forces it.