[gRPC migration] Phase 5 — Migrate other production consumers (faucet, tic-tac-toe, rpc-loadgen)

[Thoralf-M]

Part of the gRPC transaction-building migration epic #12096. Depends on Phase 1 (wallet-context gRPC support).

Goal

Move the remaining production (non-CLI) consumers off the JSON-RPC transaction
builder once WalletContext::get_grpc_client() exists.

Consumers

• faucet — crates/iota-faucet/src/faucet/simple_faucet.rs
  (~11 client.transaction_builder() calls). Migrate build_pay_iota_txn
  (currently client.transaction_builder().pay_iota(...)) to the gRPC builder.
  Raw ProgrammableTransactionBuilder batch paths can stay as-is.
• tic-tac-toe example — examples/tic-tac-toe/cli/src/client.rs
  (~3-4 calls, uses tx_data_for_dry_run). Migrate now that dry-run gas
  estimation is confirmed supported on the gRPC/SDK path.
• rpc-loadgen — crates/iota-rpc-loadgen/src/payload/pay_iota.rs.
  Lowest priority: it builds transactions with direct
  TransactionData::new_transfer_iota and does not use the JSON-RPC
  builder at all. Optional: route object reads through the gRPC client for
  consistency.

Acceptance

• faucet issues transactions via the gRPC builder; tic-tac-toe example builds +
  dry-runs via gRPC; rpc-loadgen decision documented.