feat(evidence): add table-free v3 and design guidance

harness/tht/evidence/authoring.py

@@ -694,7 +694,7 @@ def migrate_workspace_evidence(
     *,
     git_status: Callable[[Path], tuple[str, ...]] | None = None,
 ) -> EvidenceMigrationReport:
-    """Rewrite v1 Curated units as readable v2 Markdown without changing semantics."""
+    """Rewrite legacy Curated units as table-free v3 Markdown without changing semantics."""
     workspace_root = workspace_root.resolve()
     evidence_root = workspace_root / "evidence"
     _reject_dirty_authoring_state(workspace_root, git_status or _git_status)
@@ -708,10 +708,10 @@ def migrate_workspace_evidence(
     if len(documents_by_id) != len(documents):
         raise EvidencePreparationError("duplicate_evidence_id")
     migrated = tuple(sorted(
-        document.id for document in documents if document.schema_version == 1
+        document.id for document in documents if document.schema_version in {1, 2}
     ))
     unchanged = tuple(sorted(
-        document.id for document in documents if document.schema_version == 2
+        document.id for document in documents if document.schema_version == 3
     ))
     if not migrated:
         return EvidenceMigrationReport(
@@ -720,7 +720,7 @@ def migrate_workspace_evidence(
             findings=validate_workspace_evidence(workspace_root).findings,
         )
     upgraded = {
-        evidence_id: document.model_copy(update={"schema_version": 2})
+        evidence_id: document.model_copy(update={"schema_version": 3})
         for evidence_id, document in documents_by_id.items()
     }
     findings = _stage_and_apply_authoring_tree(workspace_root, upgraded, manifest)
@@ -1007,7 +1007,7 @@ def _candidate_to_evidence(
         mode="json",
         exclude={"schema_version", "existing_id", "supporting_excerpts"},
     )
-    data["schema_version"] = 2
+    data["schema_version"] = 3
     data["id"] = evidence_id
     data["provenance"] = {
         "source_file": source_file,
@@ -1028,7 +1028,7 @@ def _unsupported_unit(
         message="The current source no longer supports this Evidence unit.",
     ),)
     return evidence.model_copy(update={
-        "schema_version": 2,
+        "schema_version": 3,
         "provenance": evidence.provenance.model_copy(update={
             "source_file": source_file,
             "source_sha256": source_hash,