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

src/ZB.MOM.WW.ScadaBridge.DelmiaNotifier/README.md

@@ -52,6 +52,15 @@ WWNotifier.exe -m Z28061 -d "C:\recipes\job.nc" -w WO12345 -p PN-7788 -s 0100 -u
 - All diagnostics (per-URL attempt, status code, which URL answered, exceptions) go to **stderr** and,
   if `LogPath` is set, an appended log file — never to stdout.
 
+## Delivery semantics
+
+The `BaseUrls` failover loop is at-least-once, not exactly-once: a connection reset *after* the
+target already processed the POST, or a slow response that exceeds `TimeoutSeconds`, both look like
+an unreachable node to this client and cause a re-POST to the next URL — even though the first
+attempt may already have succeeded. The deployed `DelmiaRecipeDownload` inbound method must be
+idempotent on `(MachineCode, DownloadPath, WorkOrderNumber)` so a duplicate delivery is harmless;
+see the design doc's "Delivery semantics: at-least-once" section for details.
+
 ## Configuration — `appsettings.json`
 
 Placed next to the exe (copied to the output on build/publish):