feat: define governed tenant erasure contracts

docs/MODULE_ARCHITECTURE.md

@@ -128,6 +128,8 @@ The following contracts are the baseline API that modules can rely on:
 - bounded reference-option search provider contract
 - single-tenant and optional batched tenant summary provider contracts
 - tenant delete-veto provider contract
+- provider-neutral tenant-erasure preview, step, idempotency, and
+  reconciliation contracts in `govoplan_core.core.tenant_erasure`
 - WebUI module contribution contract
 - navigation metadata contract
 - command/event envelope contract
@@ -149,6 +151,17 @@ Destructive tenant lifecycle planning deliberately continues to use the
 single-tenant path so it invokes every registered provider for the target
 tenant, independent of ordinary list-page projections.
 
+Governed populated-tenant erasure is separate from ordinary delete vetoes.
+Modules contribute `tenancy.erasure_provider.<module_id>` capabilities with a
+bounded resource inventory, explicit erase/retain/legal-hold/external/key/
+backup dispositions, ordered destructive warnings, idempotent step execution,
+and reconciliation. The collector fails closed when a provider is invalid or
+fails. A module with nonzero tenant summary counts and no erasure capability is
+reported as unsupported and blocks execution; modules with neither contract
+are explicitly projected as outside tenant-persistence scope. Provider
+evidence contains counts and stable references only and must never contain
+secrets or erased subject data.
+
 This list is the Milestone A kernel-contract freeze baseline. New module work
 may extend the kernel by adding explicit contracts, but existing contracts must
 remain source-compatible through the 0.1.x split line unless a migration shim