Prepare browser DSP and bound 64-track startup work

[tamashi095]

Cold 64-track browser playback could compile DSP functions and greedily fill every internal source queue inside the first audio callback. Add an optional worker preparation API that returns the validated shape and retained compiled module, let the live host reuse that module with fresh DSP state, and cap each source to two accepted PCM submissions per callback. Existing shape-only scratch and URL-only host APIs remain compatible.

Prepare engine 0.2.2 and its existing OIDC release identity checks. No Rust DSP, download, decoder, or all-source readiness policy changes.

Validation: 188 engine tests and the publishable-tarball gate pass. A fresh packed adapter consumer passes initial/resumed/running PCM/runway proofs and terminal worker failure cleanup. Three fresh Chromium processes with the actual 64-track EQ/compressor app graph show all 6,785 full startup callbacks below the 2.667 ms budget; two sustained cold/warm EOF-plus-eight-seek runs preserve full 737 MB verification, four-download concurrency and zero feed errors. A targeted late-seek trace adds 1,510 callbacks below budget. Preserve one earlier nonreproduced coarse-clock telemetry outlier and the unqualified physical iPad/Safari limit; this is not a universal scheduling guarantee.

Independent Astra xhigh review: PASS. Exact archive/trace provenance, failed experiments and gates are recorded in issue 719's synchronized spec. Publication follows merge and the exact-SHA qualify → publish OIDC workflow.

Closes #719.

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