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>

docs/contributing.md

@@ -19,10 +19,11 @@ Thanks for contributing to Fusion.
 pnpm install --frozen-lockfile
 ```
 
-### Build all packages
+### Build workspace packages
 
 ```bash
-pnpm build
+pnpm build      # default build (excludes desktop/mobile)
+pnpm build:all  # full recursive build including desktop/mobile
 ```
 
 ## Workspace Package Overview
@@ -44,7 +45,8 @@ pnpm dev               # build + run CLI entrypoint in dev mode
 pnpm dev:ui            # dashboard dev server only
 pnpm lint              # lint all packages
 pnpm test              # workspace test suite (clean-worktree compatible)
-pnpm build             # workspace builds
+pnpm build             # workspace builds (excludes desktop/mobile)
+pnpm build:all         # full workspace build (includes desktop/mobile)
 pnpm verify:workspace  # canonical lint -> test -> build verification gate
 pnpm typecheck         # workspace typechecks
 ```