Stabilize nightly E2E with class isolation

[ivan-digital]

Summary

• run each selected Nightly test class in a fresh, non-parallel Swift test process while preserving matrix include and skip semantics
• remove a hosted-runner wall-clock assertion while retaining the diarization correctness assertion
• clear CosyVoice MLX cache between forward-pass tests and skip two redundant LLM micro-tests on hosted Nightly
• add a regression test for exact filtered selection and per-class invocation

Architecture and risk

This is limited to test and CI infrastructure; production APIs and runtime behavior are unchanged. The runner fails when selection is empty or invalid, streams per-class logs, continues to summarize failures, and returns a nonzero overall status.

Validation

• swift build --build-tests --disable-sandbox
• swift test --filter ShardIsolationScriptTests --disable-sandbox --no-parallel
• bash syntax, ShellCheck, Actionlint, and diff checks
• VAD Nightly: https://github.com/soniqo/speech-swift/actions/runs/32462000597 — 174 tests across 17 isolated classes
• CosyVoice Nightly: https://github.com/soniqo/speech-swift/actions/runs/32462003652 — 104 tests across 14 isolated classes

No documentation update is required because this changes internal test orchestration only.