Add a staged implementation plan for the pipeline stages

docs/roadmap/3-pipeline-stages-chunking-merge-normalize.md

@@ -81,17 +81,11 @@ The repository should also contain a resolved pipeline model that represents:
 The runner should orchestrate fake implementations through chunk, extract,
 merge, normalize, and approval/validation behavior in tests.
 
-The checkpoint should include one fixture-driven integration test that starts
-from fixture input bytes and ends at encoded output bytes. The fixture should
-use an in-memory pipeline profile with fake input adapter, deterministic
-chunker, trivial extractor, fake structured LLM client, generic merger, no-op
-normalizer, and fake or minimal JSON output encoder. This is a contract
-exercise, not a useful user-facing workflow.
-
-The walking skeleton should live in `internal/framework/pipeline`, with
-fixtures under that package's `testdata/`. If the CLI has an extract command by
-then, the same fixture may also be exercised through CLI wiring; if not, CLI
-coverage remains deferred.
+The checkpoint should include a fixture-driven walking skeleton that starts from
+fixture input bytes and ends at encoded output bytes. The walking skeleton should
+exercise the stage contracts, resolved pipeline model, module metadata,
+capability validation, defaults, lane selection, and fake LLM client wiring. It
+is contract coverage, not useful user-facing behavior.
 
 ## Design Intent
 
@@ -99,7 +93,7 @@ Chunking is a core application concern because many source documents, especially
 transcripts, will be too large for a single LLM extraction pass.
 
 Chunk processing may be serial or parallel depending on extractor needs. The
-architecture should support both, but the first implementation can execute
+architecture should support both, while checkpoint 3 may execute
 deterministically in series until a later checkpoint introduces concurrency.
 
 Merge and normalize are separate stages:
@@ -120,8 +114,9 @@ The architecture should leave room for extractor-level processing modes:
 - serial chunk processing;
 - parallel chunk processing.
 
-The first implementation may model these modes without implementing parallel
-execution. It should not bake in a single-pass assumption.
+Checkpoint 3 may execute chunks serially for deterministic behavior.
+The contracts should not bake in a single-pass assumption or prevent later
+parallel execution.
 
 ## Generic Merge Behavior
 
@@ -143,29 +138,16 @@ Domain-specific normalizers may later:
 - enforce chronological or source-reference consistency;
 - attach normalization warnings.
 
-## Walking Skeleton Fixture
+## Walking Skeleton
 
 The fixture-driven skeleton should prove the staged architecture continuously as
-new contracts are added. It should be deliberately small:
+new contracts are added. It should remain deliberately small and use fake modules
+only. It should validate module keys and flat capability requirements before
+execution, using registry metadata rather than constructing modules. Capability
+values should remain simple strings.
 
-- a fixture source document or raw source input with two or three source units;
-- a fake input adapter that parses the fixture into `SourceDocument`;
-- a deterministic chunker that produces multiple chunks;
-- a trivial extractor that calls the fake structured LLM client and emits one
-  artifact candidate per chunk;
-- a generic append-in-chunk-order merger;
-- a no-op normalizer;
-- a fake or minimal JSON output encoder;
-- an expected output fixture checked byte-for-byte or by unmarshaled structural
-  equality.
-
-The fake LLM client should be part of the test setup so the contract is
-exercised without introducing provider code, prompt assets, or response schema
-assets.
-
-The walking skeleton should validate module keys and flat capability
-requirements before execution. This should use registry metadata rather than
-constructing modules. Keep capability values as simple strings.
+Implementation staging for the walking skeleton belongs in
+[`implementation.md`](implementation.md).
 
 ## Done Criteria