The nitro server build re-bundles the whole dep tree (`noExternals: true`), so
every React package shipping a `"use client"` banner earns a MODULE_LEVEL_DIRECTIVE
warning — ~150 locally, ~800 in CI — which buries the warnings worth reading.
Ignoring the banner is correct rather than papered over: this bundle is the
Bun/Nitro server, not an RSC module graph, and TanStack Start splits client from
server with its own transform, so nothing downstream consults it.
Supplying `onwarn` replaces nitro's own handler, so its three filters
(CIRCULAR_DEPENDENCY, EVAL, "Unsupported source map comment") are restated.
Verified: `bun run build` drops from 148 such lines to 0 with no other log
delta; `tsc --noEmit` and `biome check` clean.
enricobuehler