Fund machine gas reserves with sUSDC

[ameya-deshmukh]

Summary

• fund wallet gas reserves through the faucet contract as zero-value sUSDC transfers
• seed the machine funding signer with a bounded sUSDC gas balance during deployment
• replace native-gas limits and environment variables with separate sUSDC gas-reserve limits

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• cargo build --workspace --release --locked
• sforge fmt --check
• sforge build
• bun run ../scripts/check-contract-abi.ts
• sforge test
• frontend/community frontend: bun install --frozen-lockfile, typecheck, build

Fixes SEI-409 — https://linear.app/seismic-systems/issue/SEI-409/3-faucet-fund-mock-wallet-gas-reserves-with-susdc

[linear-code]

SEI-409