feat: Linux / Raspberry Pi (ARM) support — portable sed, local ARM TTS engines, system-Chromium config

Verified on a Raspberry Pi 5 (ARM64, Python 3.13). Changes:

- new_project.sh: portable in-place sed (`-i.bak` + rm) so scaffolding works on
  GNU sed, not only BSD/macOS sed.
- tts_build.py: two extra TTS engines for Linux/ARM, both env-driven with no
  hardcoded paths and clear errors when unset:
    * kokoro_onnx — onnxruntime kokoro (no torch/spaCy; installs cleanly on
      aarch64/py3.13 where the `kokoro` engine does not) — natural voice.
    * piper — fast local fallback (onnxruntime VITS), Pi-native.
  Both slot into the existing no-word-boundary per-chunk path; cache key and the
  timeline `voice` field updated accordingly.
- remotion.config.ts: point Remotion at a system Chromium via
  REMOTION_BROWSER_EXECUTABLE (+ optional REMOTION_GL). No-op on macOS/x86, since
  Chrome-for-Testing has no linux-arm64 build.
- README: a "Linux / Raspberry Pi (ARM)" section documenting the above.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>