core: explicit CPU thread count and compute-device selection for embedders

[erichchampion]

Last of four small embedder patches from Cadenza (independent of the other three).

What it does

An embedder today can only steer GGML through the GGML_BACKEND environment variable, and cannot pick a thread count at all. Environment variables are the wrong interface for an app: a GUI cannot ask its user to export one before launch, and a mobile app has no shell at all.

This adds src/backend-config.h: a set-once, unsynchronized configuration object --

ace_backend_configure(/*device*/ "Metal", /*n_threads*/ 4);   // both at once
ace_backend_set_device(...); ace_backend_set_threads(...);     // or one field

-- that backend init reads before its first model load:

• NULL/"" device means exactly today's behaviour (GGML_BACKEND env var, then auto-best), so the CLIs are unchanged.
• A positive thread count caps GGML inference threads; <= 0 keeps the auto default.
• The device name is one ggml registers; a request that matches nothing dies with the available names printed -- and the message says whether the request came from the config API or the env var, so a misconfigured embedder is not sent debugging the wrong knob.
• The override is clamped to the logical CPU count (preferring Apple's perflevel0.physicalcpu where available, since GEMM shares SIMD units across hyperthreads), and the MP3 export encoder honours the same cap through one shared clamp helper so the two workloads cannot diverge.

Testing

tests/test-backend-config.cpp covers config read-back, clamping, and end-to-end device selection against the real registry (Metal on an Apple-silicon Mac, plus the CPU path). Built and run on macOS/arm64/Metal.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between c9045e2 and bc4c10f.

📒 Files selected for processing (5)

• src/audio-io.h
• src/backend-config.h
• src/backend.h
• tests/CMakeLists.txt
• tests/test-backend-config.cpp

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