feat(observability): replace synthetic text-turn fixture with a real sanitized flash capture (D5)

Installed a real DISPATCH_RECORD_FIXTURE capture (200 text/event-stream, deepseek-v4-flash, finish_reason stop, reply 'Hello there friend') as src/__fixtures__/flash-text-turn.json, replacing the hand-authored one. Auth header masked (Bearer sk-…redacted…UN0); fixture re-verified secret-free before commit. Text-turn replay assertions updated to real values (inputTokens 665 / outputTokens 90); structural assertions kept (multi-chunk replay, getCapturedRequest deep-equals the outgoing request, finish/stop event, concatenated deltas). The provider's request-building + SSE parsing now run against genuine flash bytes.

Real-data finding (logged in tasks.md): flash reports cache tokens via DeepSeek's NESTED prompt_tokens_details.cached_tokens (384 cached of 665 prompt); the parser only maps the FLAT cache_read/creation form, so cache tokens don't surface — an observability gap vs the §3.1 cache-debugging goal. Deferred pending decision. 334 tests, typecheck + biome 0/0.

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