Add EmitsPartials capability flag for finals-only STT providers

[a692570]

Summary

Implements the shape sketched in #75: an optional EmitsPartials() bool capability interface in internal/stt, type-asserted once in runInbound and defaulting to true, so finals-only providers get a degraded barge-in instead of none.

• stt.PartialsEmitter sits next to Client; providers that stream interims return true, finals-only providers return false, and providers that don't implement it keep the partials-driven path exactly as before.
• runInbound asserts once after stt.NewClient. When false, the backchannel candidate opens on VAD alone, and the interrupt still cannot fire until the full 600ms backchannelWindow has elapsed: a burst that ends inside the window classifies as backchannel, since there is no partial text to say otherwise. Live captions are unchanged (finals flow as before).
• Wired where needed: openaiClient returns false (batch transcription); telnyxSessionClient returns true, telnyxUtteranceClient (the exact-case in-house Telnyx engine) returns false, the same split NewTelnyxClient already uses for socket lifecycle.

Why

openai has had dead barge-in since day one and nobody wrote it down; the in-house Telnyx engine shipped the same hole. Waiting for partial text that never arrives means hasPartialText never goes true and the barge-in trigger never fires. VAD-only after the full window is the honest degraded mode: sustained interruptions work, short unclassifiable bursts stay suppressed, and nothing about any other provider changes.

Existing-provider safety

For every provider without the interface (and telnyx hosted engines), emitsPartials is true and the trigger short-circuits to the original hasPartialText.Load() expression, same bytes, same frames, same state machine. No dispatch or config schema changes; the only existing-provider edits are the EmitsPartials methods, plus rewording the telnyx startup log line and one stale comment that claimed barge-in was off.

Tests

• internal/stt: plain Client does not satisfy PartialsEmitter (the default-true contract); telnyx true/false by engine through NewTelnyxClient; openai false.
• internal/pipeline: hermetic runInbound drives: no partial text means no barge-in (current path unchanged); partials plus sustained speech fires; VAD-only holds the interrupt until the window elapses and fires after; a short burst inside the window is suppressed.
• CI green: gofmt -l ., go build ./..., go vet ./..., go test -race ./....

Docs

All "barge-in and live captions are off" claims (providers.md and zh-CN, README and zh-CN, configuration.md and zh-CN, the config.toml.example comment, the config.go doc comment, and the telnyx startup log) now describe the degraded mode instead.

Follows #78 (Telnyx STT) and option 3 from #75.