fix: geometric retry escalation to keep mempool replacement valid (#28)

The previous linear `100 + 20·N` schedule causes the new/old gas ratio
to fall below the 1.10 EIP-1559 mempool replacement floor around the
7th retry, silently triggering the `transaction-underpriced-multiplier`
50% bump path on top of the regular retry — effectively double-charging
each subsequent retry.

Geometric `1.20^N` keeps the retry/previous ratio at exactly 1.20 every
attempt, so replacement always passes by a 20% margin. Reduces redundant
retries during congestion and the resulting snowball cost.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: arpan-jain

src/executor/executor.ts

@@ -134,7 +134,12 @@ export class Executor {
         ]
 
         if (bundle.submissionAttempts > 0) {
-            const multiplier = 100n + BigInt(bundle.submissionAttempts) * 20n
+            // Geometric: keeps retry/prev ratio at 1.20; linear `100+20·N`
+            // drops below the 1.10 mempool replacement floor at N=7.
+            let multiplier = 100n
+            for (let i = 0; i < bundle.submissionAttempts; i++) {
+                multiplier = (multiplier * 120n) / 100n
+            }
 
             networkMaxFeePerGas = scaleBigIntByPercent(
                 networkMaxFeePerGas,