Seed frozen public barrels for 4 features + UI adapter

src/features/flights-map/index.ts

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+// Public barrel for the flights-map feature. See frozen-barrels.md.
+export {};

src/features/online-board/index.ts

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+// Public barrel for the online-board feature.
+// This file is the ONLY public surface — other sub-plans and features
+// must import exclusively from "@/features/online-board", never from
+// deeper paths. See docs/superpowers/phase-1/frozen-barrels.md for the rule.
+export {};

src/features/popular-requests/index.ts

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+// Public barrel for the popular-requests feature. See frozen-barrels.md.
+export {};

src/features/schedule/index.ts

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+// Public barrel for the schedule feature. See frozen-barrels.md.
+export {};

src/typecheck-placeholder.ts

@@ -1 +0,0 @@
-export {};

src/ui/index.ts

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+// Public barrel for the UI adapter layer. See frozen-barrels.md.
+// Feature code imports UI primitives exclusively through this barrel.
+export {};