TtsReader@9333480: distinguishes partial-render (yellow Warning, audio
plays, 'Re-render N failed sentences' button) from full-fail (red
Danger, 'Try render again'). New TtsFallbackChainFailedException carries
both voices when Kokoro + Piper both fail; chapter breadcrumb names
the entire chain instead of just the requested voice. +8 tests.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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