fix(bench): connection leak + faucet U256 underflow

[keanji-x]

Summary

Two production bugs surfaced during a 1500-sender long-run bench against a single reth RPC.

Bug 1 — 20K-connection leak (src/eth/eth_cli.rs)

Symptom: Target reth's rpc.max-connections=20000 cap is exhausted within minutes, wedging the RPC server. ss -t state established on the bench host shows ~20K sockets to one peer.

Root causes:

1. pool_idle_timeout(Duration::from_secs(10)) — too aggressive. reqwest's default is 90s. With 10s, sockets get evicted from the pool faster than 1500 concurrent senders can reuse them, so each request opens a new TCP connection.
2. Each hot RPC (get_pending_txn_count, send_raw_tx, get_latest_txn_count) was wrapped in tokio::time::timeout(Duration::from_secs(10), ...). When that fires, the future is dropped mid-response; reqwest treats that as an aborted request and discards the socket instead of returning it to the pool. Under any tail latency this leaks connections fast.

Fix:

• pool_idle_timeout → 90s (reqwest default)
• Add .timeout(Duration::from_secs(10)) to the reqwest::ClientBuilder chain. reqwest's built-in timeout is connection-aware and recycles the socket back to the pool when it fires.
• Remove all three tokio::time::timeout(...) wraps at the call sites.

Measured impact: 20K → 2K ESTABLISHED connections in the long-run bench.

Bug 2 — U256 underflow in faucet cascade (src/main.rs, src/txn_plan/constructor/faucet.rs)

Scenario: If fauce_eth_balance (config) is larger than the actual on-chain faucet balance, or gas_budget × total_txns exceeds fauce_eth_balance, then inside FaucetTreePlanBuilder::new:

let amount_for_leaves = faucet_balance - total_cost;  // U256 wraps silently

The U256 subtraction wraps to ~2^256, producing per-child funding values that permanently strip ENOUGH_BALANCE checks in the reth txpool — the cascade silently deadlocks with no progress and no error.

Fixes:

• src/main.rs: query eth_getBalance(faucet) at startup and compute effective_faucet_eth = min(configured, on_chain) − 1% headroom (or clamp down when config exceeds on-chain). Pass this to create_faucet_tree_plan_builder instead of the raw config value.
• src/txn_plan/constructor/faucet.rs: assert!(faucet_balance >= total_cost, ...) before the subtraction, with a diagnostic that names every input.

Drive-by — nonce-init tuning (src/main.rs)

• buffer_unordered: 1024 → 16 (don't hammer the RPC during init)
• per-call timeout: 10s → 60s (init under load is genuinely slow)

Test plan

• cargo check clean (no warnings)
• Stable across overnight 1500-sender long-run bench; ESTABLISHED socket count plateaus around 2K instead of climbing to 20K
• Cascade no longer deadlocks when fauce_eth_balance is misconfigured — assertion now fires with a readable diagnostic