Rewrite internal documentation for current stage and state contracts

docs/internal/stage-merge.md

@@ -1,63 +1,25 @@
 # Stage: merge
 
 ## Purpose
-Normalize per-speaker raw transcripts and merge them into the base transcript via Seriatim.
+Normalize raw transcript inputs and merge into base transcript via Seriatim.
 
-## Inputs and Outputs
-Inputs:
+## Inputs
 - `transcripts/raw/*.json`
 - `inputs/speakers.yml`
 - `inputs/autocorrect.yml`
 
-Outputs:
+## Outputs
 - `transcripts/base.json`
-- optional `artifacts/seriatim.report.json` (when report enabled)
+- optional `artifacts/seriatim.report.json`
 
-## Boundaries
-Owns:
-- Raw transcript discovery/validation
-- Per-input normalize calls to Seriatim
-- Final merge call to Seriatim
-- Run-local log/config/report path wiring
-- Materialization of base/report outputs to canonical paths
+## Key Behavior
+- discovers and validates raw transcript inputs.
+- normalizes each raw transcript (`seriatim.Normalize`) into run-local scratch output.
+- merges normalized inputs (`seriatim.Run`) into base transcript.
+- validates merged transcript and optional report JSON.
+- materializes canonical outputs and records stage logs/generated configs.
 
-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.
-- Materializes canonical base 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 base 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.
-- Base transcript must validate before materialization.
-- Report output is optional and gated by config.
+## Invariants
+- merge always consumes normalized forms of raw inputs.
+- base transcript must validate before stage success.
+- report output is config-gated.