docs(release-control): track governed proof assets

docs/release-control/v6/internal/SUBSYSTEM_CONTRACT_TEMPLATE.md

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+# Subsystem Contract Template
+
+Use this template for any new major subsystem that needs a canonical contract.
+
+## Contract Metadata
+
+```json
+{
+  "subsystem_id": "example-subsystem",
+  "lane": "L0",
+  "contract_file": "docs/release-control/v6/internal/subsystems/example-subsystem.md",
+  "status_file": "docs/release-control/v6/internal/status.json",
+  "registry_file": "docs/release-control/v6/internal/subsystems/registry.json",
+  "dependency_subsystem_ids": []
+}
+```
+
+## Purpose
+
+State what this subsystem owns and what it explicitly does not own.
+
+## Canonical Files
+
+List the files that contain the subsystem truth.
+
+## Shared Boundaries
+
+List any runtime files intentionally shared with other subsystems.
+Use the exact registry-derived item shape:
+`1. \`path\` shared with \`partner-subsystem\`: rationale.`
+If none exist, write `1. None.`
+
+## Extension Points
+
+List the only approved places to extend the subsystem.
+
+## Forbidden Paths
+
+List the patterns and files that future work must not use.
+
+## Completion Obligations
+
+List what must be updated when the subsystem changes.
+
+## Current State
+
+Record the current migration/end-state summary in a few lines.