Clarify user and integration documentation contracts

docs/integrations/whisperx.md

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+# Integration: WhisperX
+
+## Purpose
+
+WhisperX transcribes each prepared speaker audio file for Narratio's
+`transcribe` stage. Narratio uses an HTTP boundary and installs each successful
+response as that speaker's raw transcript JSON.
+
+## HTTP Boundary
+
+Narratio sends an HTTP `POST` to the configured transcription URL using
+`multipart/form-data` with:
+
+- `file`: the audio file, retaining its base filename; and
+- `language`: the configured language string.
+
+The server must return a `2xx` response whose body is valid JSON. Narratio does
+not currently require a more specific response schema at this boundary.
+
+## Request And Result Contract
+
+Each adapter request identifies a speaker, a readable audio file, and the
+destination for the raw transcript. The HTTP request carries the audio and
+language; the speaker identifier remains Narratio orchestration metadata.
+
+On success, Narratio atomically writes the response body to the requested
+destination. The adapter result reports that logical output together with the
+attempt count, final HTTP status when available, elapsed duration, and adapter
+identity metadata. A failed or invalid response is not installed as the
+transcript output.
+
+## Retry, Timeout, And Cancellation
+
+- The configured timeout applies independently to each HTTP attempt.
+- `retries` means additional attempts after the first.
+- HTTP `429`, HTTP `5xx`, attempt timeouts, and network errors are retryable.
+- Other HTTP `4xx` responses and explicit cancellation are not retryable.
+- Narratio waits the configured retry delay between attempts and aborts that
+  wait when the parent context is canceled.
+
+## Validation And Failure Semantics
+
+Client construction rejects a missing or invalid absolute transcription URL,
+a missing language, a non-positive timeout, negative retries, or a negative
+retry delay. A request fails before transmission when its audio or output path
+is missing.
+
+Non-`2xx` status, transport failure, response-size overflow, invalid JSON, or
+failure to install the output causes the transcription to fail. Errors include
+attempt context, and the result retains attempts, final status when available,
+and elapsed duration for diagnostics.
+
+## Determinism And Concurrency
+
+Each audio request has stable multipart field names, and successful bytes are
+installed atomically. The transcribe stage may process speaker files in
+parallel, bounded by the configured concurrency. It records results in stable
+speaker order after all work completes; any speaker failure fails the stage.
+
+## Related Canonical Docs
+
+- [Configuration](../config.md#pipeline) defines the operator-selected
+  WhisperX URL, language, timeouts, retry policy, and concurrency.
+- [Adapter implementation](../internal/adapters.md) describes internal wiring.
+- [Transcribe stage](../internal/stage-transcribe.md) describes stage mechanics,
+  durable artifacts, and manifests.