Context

buzz-voice is reusable Rust code, but the published sherpa-onnx-sys build rejects iOS and Android targets before a client can supply the official native libraries. Mobile clients also need an interruptible synthesis contract so session cancellation does not wait for a full utterance.

Summary

This PR adds explicit static and shared link modes, a backwards-compatible cancellation callback for the April Pocket engine, and a provenance-preserving sherpa-onnx-sys patch that links caller-supplied official mobile libraries through SHERPA_ONNX_LIB_DIR.

No model or native binary is bundled.

Changes

• Keeps static as the default buzz-voice link mode and adds an explicit shared mode.
• Preserves synth_chunk and adds synth_chunk_with_callback(text, lang, style, steps, Option<F>).
• Polls cancellation during April latent generation, reports monotonic progress across split text chunks, rejects same-engine callback re-entry, and converts callback panics into errors without poisoning the engine mutex.
• Carries the upstream sherpa-onnx-sys 1.13.4 metadata, bindings, license, and Cargo.toml.orig unchanged.
• Adds fail-closed iOS static and Android shared linking for caller-supplied official libraries.
• Keeps the patched sys package outside cargo-chef's version-masked workspace recipe and copies its real source into both Docker dependency builders.

Related issue

None found.

Testing

• cargo fmt --all -- --check: passed.
• cargo test --locked -p buzz-voice --lib: 8 passed, 4 model-dependent tests ignored.
• Static and shared host checks: passed.
• Static and shared clippy with -D warnings: passed.
• cargo package --locked --offline --allow-dirty -p buzz-voice: passed.
• Root and Desktop locked metadata: passed.
• Rust target checks passed for aarch64-apple-ios, aarch64-apple-ios-sim, aarch64-linux-android, and x86_64-linux-android through the fail-closed caller-library interface.
• The prior package implementation produced real non-silent 24 kHz PCM on both iOS and Android in Berd Mobile draft PR Add desktop Home feed #12. Fresh official-library linking and PCM against the April engine remain required when downstream consumers repin to this head.
• Focused NCMB, Stacksmith, Kalvin, minimize-diff, timeless-comment, and dead-code reviews are clean after fixing callback progress, re-entry, and panic handling.
• DCO trailers are present on both commits.

Screenshots

N/A. This is a Rust API and native-link packaging change with no UI.

Reviewer-reproducible examples

Run the callback and April engine tests:

cargo test --locked -p buzz-voice --lib

Exercise both host link modes:

SHERPA_ONNX_ARCHIVE_DIR=/absolute/path/to/official/archives \
  cargo check --locked -p buzz-voice --all-targets

SHERPA_ONNX_ARCHIVE_DIR=/absolute/path/to/official/archives \
  cargo check --locked -p buzz-voice --all-targets \
  --no-default-features --features shared

With official sherpa-onnx 1.13.4 libraries prepared by the caller:

SHERPA_ONNX_LIB_DIR=/absolute/path/to/iphoneos-arm64 \
  cargo check --locked -p buzz-voice --target aarch64-apple-ios \
  --no-default-features --features static

SHERPA_ONNX_LIB_DIR=/absolute/path/to/arm64-v8a \
  cargo check --locked -p buzz-voice --target aarch64-linux-android \
  --no-default-features --features shared