QVAC-25109 feat[asr-ggml]: propagate Unified RNN-T cache-aware streaming

[freddy311082]

What

Propagates the Unified RNN-T cache-aware streaming that speech-cpp gained in
qvac-fabric-speech.cpp#258
up to @qvac/asr-ggml.

parakeet-unified-en-0.6b no longer re-encodes a sliding window while
streaming: the engine keeps per-layer attention and convolution caches across
steps, so the chunk cadence now selects a trained operating point instead of an
arbitrary window length.

Asana: QVAC-25109.

How

• parakeet-unified-en-0.6b gets its own streaming chunk default of 560 ms, one
  of the trained chunks, instead of the generic 2000 ms that the engine would
  snap down to 1040 ms anyway. The documented default and the effective one now
  agree.
• Out-of-set values keep being forwarded untouched, so validation and snapping
  stay in speech-cpp rather than being duplicated in the addon.
• Documents the accepted values on the driver and the parakeet options: chunks
  of 80, 160, 560 or 1040 ms, right lookahead of 0, 80, 160, 240, 320, 560 or
  1040 ms, both snapped down to the nearest trained value.
• Raises the speech-cpp floor to 2026-09-17.

Testing

New addon unit tests cover the default, the four trained chunks passing through
unchanged, that the default is itself a trained operating point, and that
out-of-set values are forwarded rather than coerced.

Full CI was run against the temporary registry and monorepo chain pinning the
ext-lib branch head, and passed end to end, including the ten prebuild
platforms and the GPU integration lanes:
https://github.com/tetherto/qvac/actions/runs/35274766020

Note for the reviewer

The speech-cpp floor in this PR only resolves once #258 merges and the
registry publishes 2026-09-17. Until then this branch's own CI will fail at
dependency resolution. The temporary vcpkg-configuration.json pointer used
for the run above is deliberately not part of this PR.

[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.

[github-actions]

License compliance — clean

No new dependency license findings in this PR.

Warn-only (shadow) mode — this check does not block merges yet.

Updated automatically by the canonical license compliance workflow.

NOTICE presence (advisory)

Missing NOTICE (advisory, does not block):

• ./docs/website
• ./packages/fabric/test/integration
• ./packages/llm-llamacpp/benchmarks/server
• ./packages/llm-llamacpp/benchmarks/performance
• ./packages/inference-addon-cpp/mobile
• ./packages/asr-ggml/benchmarks/server
• ./packages/embed-llamacpp/benchmarks/server
• ./packages/embed-llamacpp/benchmarks/performance
• ./packages/sdk/e2e
• ./packages/vla-ggml/sim/server
• ./.github/actions/release-merge-guard