Add stateless headless CLI

[nkmlombardi]

What changed

Adds a one-shot, stateless headless CLI to the desktop app:

Blockbench --headless \
  --input creature.bbmodel \
  --script transform.js \
  --output creature_modified.bbmodel

• Boots Blockbench in a hidden offscreen renderer, so scripts use the real live globals (Project, Cube, Group, Mesh, Texture, Undo, Canvas, codecs, etc.).
• Loads through Blockbench's normal model-loading path and saves .bbmodel output through Codecs.project.
• Supports async script bodies, CommonJS function exports, --eval, trailing script arguments, --force, and a configurable timeout.
• Emits a machine-readable JSON result and phase-specific process exit codes.
• Uses a unique temporary profile for every invocation and removes it after Chromium exits.
• Skips plugin loading/network calls, updates, recent-project state, backups, and other persistent desktop startup work in headless mode.
• Atomically claims non-forced output paths so concurrent actions cannot both overwrite the same destination.
• Documents the CLI and its trusted-code security boundary.

Why

Automation currently requires either raw file transformations, which bypass Blockbench's runtime behavior, or a long-lived interactive/plugin server with shared project state. This provides a direct process boundary: one input, one action against the actual renderer APIs, one output, then exit.

Independent invocations do not share Project, Undo, scene, codec, profile, port, or plugin state, so batch workers and coding agents can safely operate on different model files in parallel.

Validation

• npm run build-electron
• npx tsc --project tsconfig.json --noEmit
• npm run test-headless-cli — 5 parser/profile tests
• node --test tests/headless_cli.e2e.test.js — help, preflight failure, live-global edit/save, and script failure tests
• BLOCKBENCH_HEADLESS_CONCURRENCY=24 node --test tests/headless_cli.concurrent.test.js — 24 independent actions completed in 8.2s; same-output collision test passed
• electron-builder --dir --win --x64 --config.win.signAndEditExecutable=false
• The packaged Windows executable passed the same 4 end-to-end tests

The existing non-headless startup path remains the default and retains its normal profile, plugins, updater, backups, recent-project handling, and visible window behavior.

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