Add new x402/built-on-stellar route to routes.txt

Required to pass the CI check that validates routes.txt matches the build output.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: marcelosalloum

routes.txt

@@ -43,6 +43,7 @@
 /docs/build/apps/wallet/sep7
 /docs/build/apps/wallet/stellar
 /docs/build/apps/x402
+/docs/build/apps/x402/built-on-stellar
 /docs/build/apps/zk
 /docs/build/guides
 /docs/build/guides/archival