feat: musicgen + embeddings/rerank surfaces (ADR-0021, ADR-0022)

- NEW musicgen leaf package: blocking Generate over ACE-Step's async job
  queue (release_task -> poll query_result -> fetch file, all via
  /upstream); tolerant envelope parsing, double-encoded result handled
- NEW embeddings leaf package: EmbedModel + RerankModel as separate mints
  (two server instances on the host, llama.cpp #20085); InstructedQuery
  helper for Qwen3-style query/document asymmetry
- provider/llamaswap: /v1/embeddings + /v1/rerank clients with strict
  validation (index-ordered vectors, count mismatch and out-of-range
  index are hard errors; rerank sorted descending, minimal parser)

Co-Authored-By: Claude Fable 5 <[email protected]>

docs/adr/0021-musicgen-interface.md

@@ -0,0 +1,35 @@
+# ADR-0021: musicgen interface (blocking Generate over an async job queue)
+
+Status: Accepted (2026-07-12)
+
+## Context
+
+The llama-swap host gained ACE-Step 1.5 (full songs with vocals, seconds per
+clip warm on the reference GPU). Its API is an async job queue —
+`POST /release_task` → poll `POST /query_result` → `GET` the result file —
+unlike every other majordomo media backend, which is one blocking call.
+
+## Decision
+
+- New `musicgen` leaf package, ADR-0016→0020 conventions: `Request{Prompt,
+  Lyrics, DurationSeconds, Format, Steps, Seed}`, `Result{Audio{Data, MIME},
+  Raw}`, functional options, zero value = backend default, bytes-only.
+- **`Model.Generate` blocks, polling internally** (2s interval, ctx-bounded).
+  The one-call contract is the package's value: callers budget the whole job
+  with a context deadline exactly like imagegen/videogen, and the async
+  mechanics stay a provider detail. An async job surface (mirroring the
+  deliberately-deferred videogen one, ADR-0019) can come later if a caller
+  ever needs progress.
+- provider/llamaswap reaches ACE-Step through the `/upstream/<model>/`
+  passthrough (ADR-0020). Envelope parsing is tolerant (`data` wrapper or
+  bare array; `result` arrives as a double-encoded JSON string) and the
+  result-file URL is routed back through the same upstream. `audio_duration`
+  is the v1 param name — an unknown field degrades to default clip length
+  upstream, never an error; verify at smoke.
+
+## Consequences
+
+- Music jobs occupy the exclusive GPU group like video; callers own the
+  timeout budget (mort keeps the tool timeout under its agent ceiling).
+- The double-encoded `result` string and envelope shapes are pinned by the
+  netherstorm image build; host smoke tests are the drift defence.