Added new internal documentation

docs/internal/adapters.md

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+# Internal: Adapters
+
+## Purpose
+Describe the external adapter boundaries used by Narratio stages and app orchestration, including default runtime wiring.
+
+## Inputs and outputs
+Inputs:
+- Stage requests passed through adapter interfaces (for example transcription, merge/normalize/trim, polish, artifact generation, object-store operations, notifications).
+- Resolved config values used to construct default adapters.
+
+Outputs:
+- Adapter-specific result structs (paths, metadata, status/attempt info, duration/exit details).
+- Adapter errors returned to stage/app orchestration.
+
+## Boundaries
+Owns:
+- Transport/process/SDK details at system boundaries (`HTTP`, subprocess CLI invocation, AWS SDK calls).
+- Request/response contracts in `internal/adapters/*` packages.
+
+Does not own:
+- Stage sequencing, skip/force/resume decisions.
+- Manifest transition logic.
+- Canonical workspace path policy.
+
+## Config fields used
+Default wiring and adapter calls consume:
+- `pipeline.whisperx.*`
+- `pipeline.seriatim.*`
+- `pipeline.audita.*`
+- `pipeline.scriptorium.*`
+- `pipeline.storage.*` and `pipeline.archive.*` (object-store construction/gating)
+- `pipeline.notification.*` (sender boundary exists; placeholder behavior today)
+
+## External adapters used
+Runtime env boundary fields (`internal/stage.Env`):
+- `whisperx.Client`
+- `seriatim.Runner`
+- `audita.Runner`
+- `scriptorium.Runner`
+- `storage.ObjectStore`
+- `notify.Sender`
+- `analyzer.Runner`
+
+Current execution usage:
+- Actively used by implemented stages: `WhisperX`, `Seriatim`, `Audita`, `Scriptorium`, `ObjectStore`, `Notifier`.
+- Present but not used by implemented stage set: `Analyzer`, legacy `storage.Backend`.
+
+Default construction in app runner:
+- Auto-constructed when not injected: WhisperX HTTP client, Seriatim subprocess runner, Audita subprocess runner, Scriptorium subprocess runner, object store (only when needed), and `notify.NoopSender`.
+- Callers can inject test/fake implementations through `app.RunOptions.Env`.
+
+## State and manifest behavior
+- Adapters do not directly mutate session/run manifests.
+- Stages and runner own manifest writes and stage status transitions.
+- Adapter outputs are persisted indirectly through stage result mapping (outputs/logs/generated configs/metadata).
+
+## Skip and resume behavior
+- No adapter-level skip/resume semantics.
+- Skip/resume/force behavior is decided by app runner using manifest stage state.
+
+## Failure behavior
+- Adapter constructors validate config-derived values and fail early on invalid required inputs.
+- Adapter run-time failures are returned to stage code with boundary context and are recorded as stage failures by runner logic.
+- Subprocess adapters preserve stdout/stderr and generated-config paths to aid diagnosis.
+
+## Tests to inspect before changing
+- `internal/adapters/whisperx/http_test.go`
+- `internal/adapters/seriatim/subprocess_test.go`
+- `internal/adapters/audita/subprocess_test.go`
+- `internal/adapters/scriptorium/subprocess_test.go`
+- `internal/adapters/storage/*_test.go`
+- `internal/adapters/notify/fake_test.go`
+- `internal/adapters/analyzer/fake_test.go`
+- `internal/app/runner_test.go`
+
+## Architectural invariants
+- Stage code depends on adapter interfaces, not transport-specific implementation types.
+- External SDK-specific types remain inside adapter implementations.
+- Default app wiring must remain deterministic and overrideable via injected env dependencies.