v0.4.0

No defect fixes. Two additions, one of them the first proof of something that
had only ever been asserted.

generate_sound_effect is now proven built, end to end

It has always registered, and its provider layer was thoroughly tested —
request contract, credit safety, undocumented response shapes, URL recognition,
retrieval discovery. The tool was tested by nothing, so "it registers" was
the only claim anyone could honestly make about it.

It is now driven end to end with the provider stubbed and everything else real:
the spend ledger, the job store, the bounded poll, the HTTPS download, the
atomic write, the workspace layout, asset.json, and the JSON a client
receives. Five properties, each falsified against a reverted fix:

• clips land on disk carrying the bytes the server sent, not just a path in
  a response
• asset.json records prompt, provider, model and generation id — a clip
  nobody can trace back to a prompt is what this server exists to prevent
• a PENDING poll is retried rather than abandoned
• a provider failure becomes a failed job, not a thrown call: a spent credit
  with a traceable record beats an exception with none
• the spend ceiling is charged before the provider is contacted, and moving
  that charge after the call is caught by this test and nothing else

⚠ Still not a substitute for one live call. Leonardo documents the sound-effect
request contract but not its response shape, so the parsing in
providers/audio/leonardo.ts remains unverified against the real API and is
marked as such below. What is proven is that everything around it is wired up.

The test harness now accepts context overrides, so any provider-backed tool can
be exercised this way without a credential.

A Features section in the README

A capability inventory rather than a tool list: what the pipeline does, what the
offline half answers, the glTF correctness the tools enforce on your behalf, the
safety properties (and that most of them exist because they were once wrong),
spend control, provenance, and how it fails. Every number in it was checked
against the running server rather than recalled.