chore(build,test): split desktop/mobile from default build; share vitest worker budget

- pnpm build now excludes @fusion/desktop and @fusion/mobile by default
  (recursive build still available as pnpm build:all). Saves time on
  workspace-wide builds that don't need the native shells.
- Hoist the per-package max-worker computation into a shared
  packages/core/src/__test-utils__/vitest-workers.ts util. Every
  vitest.config.ts now calls computeMaxWorkers(), which honors
  VITEST_MAX_WORKERS, FUSION_TEST_TOTAL_WORKERS, and a per-config
  defaultCap, clamped to cpus-1.
- pnpm test sets VITEST_MAX_WORKERS=2 so the workspace run keeps total
  fan-out modest with --workspace-concurrency=2.
- Switch dashboard vitest pool from forks to threads so jsdom/React
  suites share a V8 heap instead of duplicating ~500MB per worker.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/vitest.config.ts

@@ -1,14 +1,8 @@
 import { defineConfig } from "vitest/config";
 import { resolve } from "node:path";
-import { cpus } from "node:os";
+import { computeMaxWorkers } from "./src/__test-utils__/vitest-workers";
 
-// Keep worker fan-out conservative by default. Over-subscribing threads can
-// starve Vitest's worker RPC channel and trigger flaky "onTaskUpdate" timeouts
-// under heavy SQLite test load.
-const defaultMaxWorkers = Math.min(4, Math.max(1, cpus().length - 1));
-const requestedMaxWorkers = Number.parseInt(process.env.VITEST_MAX_WORKERS ?? String(defaultMaxWorkers), 10);
-const maxWorkers = Math.max(1, Number.isFinite(requestedMaxWorkers) ? requestedMaxWorkers : defaultMaxWorkers);
-process.env.VITEST_MAX_WORKERS = String(maxWorkers);
+const maxWorkers = computeMaxWorkers();
 
 export default defineConfig({
   resolve: {