feat: start mailbox evidence scanner

docs/email-evidence-canon.md

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+# Email Evidence Canon
+
+## Rule
+
+Email events are evidence, not result satisfaction.
+
+The scanner reports email-channel facts and uncertainty. A downstream
+coordination runtime decides whether those facts satisfy a coordination case.
+
+## Initial Message Classes
+
+- `hard_bounce`
+- `soft_bounce`
+- `delayed_delivery_notice`
+- `final_delivery_failure`
+- `out_of_office`
+- `human_reply`
+- `complaint_or_abuse`
+- `unsubscribe_or_opt_out`
+- `challenge_response`
+- `unknown_return_message`
+- `unrelated_message`
+- `parse_failed`
+
+## Initial Normalized Events
+
+| Message class | Normalized event | Assessment |
+| --- | --- | --- |
+| `hard_bounce` | `notification.endpoint.rejected_permanent` | `fail.hard_bounce` |
+| `soft_bounce` | `notification.endpoint.rejected_temporary` | `undef.deferred` |
+| `delayed_delivery_notice` | `notification.endpoint.deferred` | `undef.deferred` |
+| `final_delivery_failure` | `notification.endpoint.rejected_permanent` | `fail.expired_without_delivery` |
+| `out_of_office` | `interaction.out_of_office_received` | `undef.out_of_office` |
+| `human_reply` | `interaction.reply_received` | `success.reply_received` |
+| `complaint_or_abuse` | `notification.channel.complaint_received` | `fail.complaint_received` |
+| `unsubscribe_or_opt_out` | `notification.channel.unsubscribe_received` | `fail.unsubscribed` |
+| `unknown_return_message` | `notification.endpoint.unknown` | `undef.conflicting_evidence` |
+| `challenge_response` | `interaction.unverified_actor_interaction` | `undef.identity_uncertain` |
+
+## Overclaim Prevention
+
+- No bounce found does not mean delivery success.
+- Provider acceptance does not mean endpoint acceptance.
+- Endpoint acceptance does not mean inbox placement.
+- Out-of-office does not prove recipient awareness or action.
+- Human reply does not prove legal acceptance.
+- Unknown return messages remain visible.
+- Scanner and proxy interactions must stay below identity-bound interaction.