feat(desktop): upgrade Pocket TTS model

[johnmatthewtennant]

Context

Buzz Desktop currently installs an older Pocket TTS model bundle. The current
bundle changes the tokenizer, learned BOS input, recurrent-state contract, and
prompt behavior, so updating download URLs alone is not compatible.

Summary

This PR upgrades Buzz Desktop to the current pinned Pocket TTS model. It
preserves existing product behavior and the hard 50-token model-input limit
while adding the required runtime support, verified acquisition, and crash-safe
cache migration.

Changes

• Pins an immutable Pocket TTS revision, artifact names, exact byte sizes,
  SHA-256 checksums, Mary reference voice, and license.
• Loads the bundle-matched SentencePiece tokenizer, learned BOS embedding, and
  bundle-declared recurrent states.
• Uses one pinned Pocket TTS configuration; no precision or model-version
  selector is added.
• Preserves the resident engine's exact <= 50 token contract without changing
  Desktop segmentation policy.
• Bumps the Pocket cache manifest to v4, verifies size and checksum before
  adoption, atomically swaps the cache, and recovers the last verified cache
  after interrupted installs, including an incomplete final directory.
• Keeps acquisition, cache migration, worker adoption, and tests within the
  existing Desktop implementation.
• Removes the obsolete model-quality harness, which was coupled to the
  superseded production prompt and model layout.

Related issue

None.

Testing

Manual listening completed on the exact Desktop build. The updated model improved speech quality and resolved the phrase-start and sample-onset artifacts. Reproducible integrity and model checks are below.

Screenshots

N/A. This changes model installation and speech synthesis, not a visual surface.

Reviewer-reproducible examples

Before and after model identity

git show 35305bfc8fd456ca9a17caa1ddbfaabd87d46981:desktop/src-tauri/src/huddle/models.rs \
  | grep -E 'sherpa-onnx-pocket-tts|TTS_MODEL_VERSION'

git show 211d17c58567448fe7ac95c4fa0ad2b88378849a:desktop/src-tauri/src/huddle/pocket_models.rs \
  | grep -E 'MODEL_REPOSITORY|MODEL_REVISION|MODEL_PRECISION|MAX_TOKENS'

The target branch identifies the January bundle. The PR branch identifies the
immutable April revision, INT8 precision, and 50-token maximum.

Deterministic runtime validation

Use the pinned artifacts listed in pocket_models.rs and run the model-dependent Pocket tests with the model directory supplied by the test environment. The checked-in long-sentence fixture must preserve its expected 48 and 44 token split and produce non-silent PCM.

Manual listening validation

John listened to an untrimmed Pocket TTS onset-stress clip generated from the exact user-provided passage, with every sentence synthesized separately and identical 100 ms digital-silence boundaries. The clip used no leading period, onset trimming, gain adjustment, or loudness normalization.

The updated model produced better-quality speech and resolved the start-of-sample artifacts.