docs(delmia): document at-least-once failover semantics + DelmiaRecipeDownload idempotency requirement

Manual verification (task step 3) skipped: this worktree cannot reach a
live environment to fetch the deployed DelmiaRecipeDownload script via
`scadabridge api-method get`. Documents only the mandated contract per
PLAN-06 Task 23; verifying the deployed script against it is deferred to
whoever has live environment access.

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

docs/requirements/Component-InboundAPI.md

@@ -47,6 +47,13 @@ Each API method definition includes:
 - **Implementation Script**: C# script that executes when the method is called. Stored **inline** in the method definition. Follows standard C# authoring patterns but has no template inheritance — it is a standalone script tied to this method.
 - **Timeout**: Configurable per method. Defines the maximum time the method is allowed to execute (including any routed calls to sites) before returning a timeout error to the caller.
 
+> **Authoring note — idempotent methods for at-least-once callers**: some external callers retry
+> on their own connect-failure/timeout failover loop and cannot tell "the first attempt actually
+> succeeded" from "the first attempt was lost" — e.g. the `DelmiaRecipeDownload` method backing
+> `DelmiaNotifier` (see `docs/plans/2026-06-26-delmia-recipe-notifier-design.md`, "Delivery
+> semantics: at-least-once"). Implementation scripts for such methods must be idempotent on their
+> natural key so a duplicate delivery returns the same result without double-applying side effects.
+
 ### Management
 - Managed by users with the **Designer** role via the Central UI.
 - All method definition changes are audit logged.