fix: avoid concurrent map writes in ProcessResult

[mmsqe]

loadtest sometimes fails due to fatal error: concurrent map writes from ProcessResult

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Note

Medium Risk
Touches concurrent metrics collection logic and adds synchronization/closure fixes; low functional surface area but mistakes could still skew load test results or reintroduce data races under high concurrency.

Overview
Prevents fatal error: concurrent map writes in ProcessResults by guarding shared receipts map writes with a mutex and by passing per-iteration values (blockNum, precomputed index) into goroutines to avoid loop-variable capture issues.

Fixes a similar goroutine capture bug in concurrent receipt fetching (getReceiptsForBlockTxs) and adds a stress-style test (TestProcessResultsConcurrent) using a simulated Ethereum backend to exercise concurrent receipt collection repeatedly. Updates module deps to include golang.org/x/sync directly and refreshes indirect dependency versions.

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