PTG v0.3.1 — Setup & serve phases

v0.3.1 — Setup & serve phases

A focused follow-on to v0.3.0: the released ptg binary can now prepare its own
runtime environment, so running a mesh is a one-command affair rather than a
manual server/model setup.

What's new

ptg setup --yes    # detect llama-server, download the gated Gemma QAT model (~2.7 GB), write config
ptg serve          # launch the inference server in the foreground (config is remembered)
ptg --probe        # then just run a mesh — no --vllm-url / --model flags needed

• ptg setup detects llama-server (flag → PTG_LLAMA_SERVER →
  LLAMA_SERVER → ~/.cache/ptg/bin → ~/llama-spike/.../build/bin → PATH),
  downloads the verified Gemma QAT model into the cache, and writes
  ~/.config/ptg/config.toml (%APPDATA% on Windows). Model download prefers
  the hf / huggingface-cli tool (handles Gemma gating best) and falls back
  to a native resumable download (HF_TOKEN bearer auth, .part → rename) if
  the CLI is absent or broken. The token is used only for the download and is
  never persisted.
• ptg serve reads the config, short-circuits if the server is already
  running, else launches llama-server in the foreground with the exact verified
  flags. --dry-run prints the command. It does not daemonize.
• Config-aware run path: the mesh command (ptg with no subcommand) now
  resolves server URL and model as flag → env → setup config → bundled default,
  so "setup once, then just ptg" works. The subcommand is optional, so all
  existing invocations are unchanged.

Honest prerequisites (documented in the README, not hidden)

• llama-server must be installed. ptg setup detects it and prints exact
  install instructions if missing, but deliberately does not build it
  (GPU/platform-specific). Install from llama.cpp.
• Gemma is gated. Accept the license at the model's HuggingFace page, then
  authenticate (hf login or set HF_TOKEN).

Validation

• 114 unit/integration tests pass; clippy/doc clean; panic-free (-D warnings
  with panic/unwrap/expect denies).
• Live-tested locally: setup --dry-run (detects the real server), serve --dry-run (exact server flags), config-aware run, and a real native
  download (broken hf shim → graceful fallback → file downloaded + renamed).
• The release workflow now smoke-tests ptg setup --dry-run on all four shipped
  targets (Linux, macOS arm64/x86_64, Windows), gating packaging on the setup
  command actually running on every platform.

No research changes

This release adds tooling only. The benchmarked structured-lateral findings and
the runtime concurrency fix from v0.3.0 are unchanged.