docs(onboard): BDD scenarios + architecture for the ADR-028 burst

- connect_account.feature: refine the burst scenario to "best recent" (likely-good
  selection, skips too-short/too-long) and add a scenario for the stronger model.
- scenario_coverage: remap to TestOnboardBurstVideoIDs + add the model scenario
  (the old NewestUnsummarizedVideoIDs test was removed).
- architecture.md: new "Connect-time onboarding burst" subsection — junk-avoiding
  selection over persisted duration, the burst-only stronger-model chain, and why
  has-captions/cached-first are not selection signals.

test/acceptance/scenario_coverage_test.go

@@ -38,10 +38,11 @@ var scenarioCoverage = map[string]string{
 	"An authenticated user on the welcome page sees their way in and out": "TestWelcomeLoggedIn",
 
 	// connect_account.feature
-	"Connect a YouTube account":                          "TestCallbackExchangesAndRecordsConnection",
-	"Connecting an account discovers videos immediately": "TestCallbackTriggersDiscovery",
-	"Connecting summarizes my newest videos right away":  "TestNewestUnsummarizedVideoIDs",
-	"Tokens are never stored in the clear":               "TestCallbackExchangesAndRecordsConnection",
+	"Connect a YouTube account":                              "TestCallbackExchangesAndRecordsConnection",
+	"Connecting an account discovers videos immediately":     "TestCallbackTriggersDiscovery",
+	"Connecting summarizes my best recent videos right away": "TestOnboardBurstVideoIDs",
+	"The onboarding burst summarizes with a stronger model":  "TestBurstChainModelsLeadsWithOnboardModel",
+	"Tokens are never stored in the clear":                   "TestCallbackExchangesAndRecordsConnection",
 
 	// paste_url.feature
 	"Paste a valid YouTube URL":                              "TestPasteValidURLAddsAndRequests",