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>

README.md

@@ -446,7 +446,8 @@ fn skills install firebase/agent-skills               # Install agent skills
 
 ```bash
 pnpm install                  # Install dependencies
-pnpm build                    # Build all packages
+pnpm build                    # Build default workspace packages (excludes desktop/mobile)
+pnpm build:all                # Build all packages (including desktop/mobile)
 pnpm dev dashboard            # Run dashboard + AI engine
 pnpm dev:ui                   # Dashboard only (no AI engine)
 pnpm lint                     # Lint all packages