feat: audio surfaces (TTS + transcription), imagegen.Editor, llamaswap health probe

- New leaf package `audio` (ADR-0017): SpeechModel/SpeechProvider and
  TranscriptionModel/TranscriptionProvider with imagegen conventions
  (zero value = backend default, functional options + Apply, bytes
  in/out, never URLs). Root re-exports added.
- imagegen.Editor (ADR-0018): optional image-to-image interface —
  EditRequest carries the generation knobs plus Init image and
  denoising Strength; separate interface so existing Models keep
  compiling.
- provider/llamaswap implements all of it: POST /v1/audio/speech (JSON,
  raw-audio response, MIME from Content-Type with format fallback),
  POST /v1/audio/transcriptions (multipart, response_format=json),
  ListVoices (GET /v1/audio/voices?model=, tolerant of string-list and
  object-list shapes), POST /sdapi/v1/img2img (txt2img wire +
  init_images/denoising_strength, shared image decode), and Health(ctx)
  (GET /health) — a cheap liveness probe for often-offline hosts.
- Hermetic httptest coverage for every new wire shape and validation
  path; README sections + support-matrix footnote updated in the same
  commit (also corrects the stale /v1/images/generations claim — the
  image path has been SDAPI since the seed fix).

First consumer: mort's llamaswap media tool cluster (status / image /
TTS / STT agent tools against the netherstorm host).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AXQxVhXBw8PwFAtsVrXSmj

docs/adr/README.md

@@ -20,3 +20,5 @@ One decision per file, append-only; supersede rather than rewrite.
 | [0014](0014-conversion-driven-extensions.md) | Conversion-driven extensions (resolvers, typed tools, hooks, ops controls) | Accepted |
 | [0015](0015-llama-swap-provider.md) | llama-swap provider — reuse openai for chat, tailored management + image | Accepted |
 | [0016](0016-imagegen-interface.md) | imagegen — a canonical text-to-image interface | Accepted |
+| [0017](0017-audio-interfaces.md) | audio — canonical speech synthesis + transcription interfaces | Accepted |
+| [0018](0018-imagegen-editor.md) | imagegen.Editor — image-to-image as a separate optional interface | Accepted |