QVAC-23458 fix: make AOSC final event deterministic

[Alok-Ranjan23]

Summary

• always emit one synthetic Sortformer final terminator after draining any trailing audio
• keep real tail segments non-final and preserve cancellation and repeated-finalize behavior
• add fixture-backed and model-independent regression coverage for tail-present, no-tail, duplicate-finalize, and cancelled sessions
• update the public AOSC contract and remove the stale nondeterministic-final-event limitation

Validation

• git diff --check
• Model-independent Sortformer finalization state test
• C++17 syntax check for test_sortformer_streaming.cpp
• C++17 syntax check for the public diarization header
• Trailing-newline checks for all edited files
• Source CI compiles the fixture-backed regression and executes the model-independent unit test on every PR
• Downstream QVAC ASR validation built all nine platform prebuilds and passed C++ coverage plus Linux/Windows/macOS integration jobs: https://github.com/tetherto/qvac/actions/runs/31521580761
• Model-backed AOSC default and disabled-path tests passed on Linux x64: https://github.com/tetherto/qvac/actions/runs/31521580761/job/93882402709
• Model-backed Sortformer source test remains fixture-gated because the v2.1 GGUF is not staged in source CI

The downstream run's relevant build and AOSC tests passed. Its overall result
was not green because the mobile lanes could not access the separate
qvac-test-addon-mobile repository, while the macOS arm64 lane hit unrelated
existing EOU assertions and an unsupported Metal CTC operation. The remaining
queued legacy Linux GPU lane was cancelled after the relevant matrix had
completed.

Downstream validation

• Registry publication: QVAC-23458 bump[parakeet-cpp]: publish deterministic final event qvac-registry-vcpkg#303
• QVAC consumer validation: QVAC-23458 fix: consume deterministic Sortformer final event qvac#3777

Task

QVAC-23458

[github-actions]

Review Status

Current Status: ❌ PENDING
Approvals so far: none

Pending reviews: Needs 1 Management or Team Lead, and 1 more from Management, Team Lead, or Member.

[Alok-Ranjan23]

Addressed in ef489ebb:

• the fixture-backed no-tail path now requires n_real_tail == 0
• finalization sequencing is extracted into a model-independent state machine
• test-sortformer-finalize runs in the non-GPU source CI lane on every PR and covers tail, no-tail, duplicate-finalize, and cancellation paths
• the source CI matrix is green

Downstream validation is linked through:

• registry publication: QVAC-23458 bump[parakeet-cpp]: publish deterministic final event qvac-registry-vcpkg#303
• QVAC consumer branch: QVAC-23458 fix: consume deterministic Sortformer final event qvac#3777
• validation run: https://github.com/tetherto/qvac/actions/runs/31521580761
• successful model-backed AOSC job: https://github.com/tetherto/qvac/actions/runs/31521580761/job/93882402709

The exact source revision built successfully in all nine prebuild targets, C++
coverage passed, and the Linux x64 model-backed AOSC default and disabled-path
tests passed. The full run was not green only because the mobile jobs could not
access the separate qvac-test-addon-mobile repository and macOS arm64 hit
unrelated existing EOU/Metal failures; those limitations are called out in all
three PR descriptions.