core: widen the cancel callback into an AceProgress report+cancel callback

[erichchampion]

Third of four small embedder patches from Cadenza (independent of the others; ported onto current master's heterogeneous LM batching from #9761469 and the dropped DiT padding-mask plumbing from #da5bc90).

What it does

The pipelines took bool (*cancel)(void *) + void * cancel_data. The same callback now also reports where the run is:

struct AceProgress {
    bool (*fn)(void * data, AceStage stage, int step, int total) = nullptr;
    void * data                                                  = nullptr;
};

Every pipeline poll calls fn with the current stage and loop position, and returning true still cancels -- so a UI gets a real progress bar over the exact hook it already had to implement anyway. The HTTP server's /progress poll-with-cancel is the visible beneficiary: today a client can cancel but cannot know whether the run is in LM, DiT or VAE, or how far along.

• Passed by value, defaulted to {} (no callback = no progress and never cancel), so every existing caller that passed nothing is unchanged, and callers that passed a cancel pair keep identical cancellation semantics.
• Stages: LM, DiT, VAE encode, VAE decode, MP3. step is 0-based, and a step-0 report fires before a loop's first unit of work, so a segmented bar can be sized even for empty loops.
• Thread-safety is documented: fn is called sequentially for LM, DiT and VAE, but MP3 encode is fork-join, so fn may be called concurrently there -- safe for the flag-reading implementations that are the common case.

Testing

tests/test-progress.cpp exercises the report/cancel contract: report sequence, cancel mid-loop, and the step-0-before-first-work guarantee. Built and run on macOS/arm64/Metal.

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📥 Commits

Reviewing files that changed from the base of the PR and between c9045e2 and 2378347.

📒 Files selected for processing (19)

• src/audio-io.h
• src/dit-sampler.h
• src/pipeline-lm.cpp
• src/pipeline-lm.h
• src/pipeline-synth-ops.cpp
• src/pipeline-synth-ops.h
• src/pipeline-synth.cpp
• src/pipeline-synth.h
• src/pipeline-understand.cpp
• src/pipeline-understand.h
• src/progress.h
• src/vae-enc.h
• src/vae.h
• tests/CMakeLists.txt
• tests/test-progress.cpp
• tools/ace-lm.cpp
• tools/ace-server.cpp
• tools/ace-understand.cpp
• tools/synth-batch-runner.h

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