Complete documentation rebuild

docs/internal/stage-merge.md

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+# Stage: merge
+
+## Purpose
+Normalize per-speaker raw transcripts and merge them into one merged transcript via Seriatim.
+
+## Inputs and Outputs
+Inputs:
+- `transcripts/raw/*.json`
+- `inputs/speakers.yml`
+- `inputs/autocorrect.yml`
+
+Outputs:
+- `transcripts/merged.json`
+- optional `artifacts/seriatim.report.json` (when report enabled)
+
+## Boundaries
+Owns:
+- Raw transcript discovery/validation
+- Per-input normalize calls to Seriatim
+- Final merge call to Seriatim
+- Run-local log/config/report path wiring
+- Promotion of merged/report outputs to canonical paths
+
+Does not own:
+- Transcript polishing or downstream artifact generation
+
+## Config Fields Used
+- `session.session_id`
+- `session.campaign`
+- `pipeline.workspace.root`
+- `pipeline.seriatim.binary`
+- `pipeline.seriatim.timeout`
+- `pipeline.seriatim.output_schema`
+- `pipeline.seriatim.coalesce_gap`
+- `pipeline.seriatim.report`
+- `pipeline.seriatim.env.*`
+
+## External Adapters Used
+- Seriatim adapter:
+  - `Normalize` for each raw input
+  - `Run` for final merge
+
+## State and Manifest Behavior
+- Reads transcript inputs from transcribe stage outputs in manifest when present; falls back to canonical raw directory.
+- Writes run-local outputs/logs/config under `runs/{run_id}/merge/...` when enabled.
+- Promotes canonical merged transcript and optional report.
+- Records normalized-input provenance and adapter metadata in stage metadata.
+
+## Skip and Resume Behavior
+- Runner-level skip applies when already succeeded and not forced.
+- Forced rerun of this or upstream stages can stale downstream succeeded stages via runner invalidation.
+
+## Failure Behavior
+- Fails on missing/invalid raw transcripts, missing speakers/autocorrect files, normalize failure, merge failure, invalid merged output JSON, or invalid report JSON when enabled.
+
+## Tests to Inspect Before Changing
+- `internal/stage/merge_test.go`
+- `internal/adapters/seriatim/subprocess_test.go`
+
+## Architectural Invariants
+- Merge consumes normalized forms of each raw transcript.
+- Merged transcript must validate before promotion.
+- Report output is optional and gated by config.