docs(adr-102): document the intentional two-validator layering (P6c)

api/app/lib/backup_integrity.py

@@ -9,9 +9,21 @@
 - External dependencies (concept references not contained in this backup — partial backups)
 - Statistics (record counts surfaced for logging)
 
-This is the *runtime* checker. The exhaustive *offline* oracle is
-``scripts/development/lint/lint_backup.py`` (used in tests/linting). Consolidating
-the two v2 validators is tracked for ADR-102 P6.
+This is the *runtime* checker, deliberately distinct from the offline oracle
+``scripts/development/lint/lint_backup.py``. They validate DIFFERENT layers and are
+intentionally NOT merged (evaluated and decided in ADR-102 P6 — do not "consolidate"
+them):
+  - THIS module checks the DE-INTERNED logical graph via :class:`KgBackupV2Reader`
+    (referential integrity, external deps, statistics) — the cheap runtime gate run
+    before a backup is streamed / archived / restored.
+  - ``lint_backup`` checks the RAW interned on-disk structure (dictionary index
+    ranges, interning integrity, §6 derived-product exclusions, format negotiation)
+    and carries NO api-package dependency by design (ADR-102 Track D), so the
+    pytest oracle can load it standalone by file path.
+
+A shared core would force one layer's representation onto the other and couple the
+standalone oracle to the api package; the narrow overlap (reference / external-dep
+checks) does not justify the cost.
 """
 
 from typing import Dict, List, Optional, Any, Set

scripts/development/lint/lint_backup.py

@@ -16,6 +16,17 @@
 wrapper that reads a path, calls the core, prints issues, and exits non-zero if
 any ERROR was found (CI convention, matching the sibling lint tools).
 
+Relationship to the runtime checker (ADR-102 P6 — intentionally NOT merged):
+this is the *independent* verification tool — a deliberately standalone oracle
+with NO api-package dependency, which is what makes it a convenient CI / test /
+build gate and an outside check on the serializer. It validates the RAW interned
+on-disk structure (dictionary index resolution, interning integrity, §6 exclusions,
+format negotiation). Its runtime counterpart, ``api/app/lib/backup_integrity.py``,
+validates the DE-INTERNED logical graph via :class:`KgBackupV2Reader` as the cheap
+gate before stream/archive/restore. The two check different layers for different
+consumers; consolidating them would couple this oracle to the api package and
+strip its interning-layer coverage, so they are kept separate by design.
+
 Checks implemented (see ``CHECK_CODES`` for the stable code registry):
   - HEADER presence / well-formedness and format_version family negotiation (§7)
   - kg-backup/2 required header fields (§3.1)
@@ -34,7 +45,7 @@
     python3 scripts/development/lint/lint_backup.py <path-to-archive.tar.gz>
     python3 scripts/development/lint/lint_backup.py --selftest
 
-@verified cffa180b
+@verified b832d59d
 """
 
 import argparse