fix: flashblock and block time settings

[meyer9]

Description

Fixes flashblock and block time settings so the benchmark harness respects configurable block times instead of hardcoding 1-second blocks.

Changes

• Configurable block time: Add top-level block_time field to BenchmarkConfig (parsed as a Go duration, default "1s"). Plumbed through ResolveTestRunsFromMatrix → RunParams.BlockTime → consensus client and rollup config.
• Configurable flashblocks block time: Add flashblocks.block_time config field (default "250ms"). Passed through RuntimeConfig to the builder client as --flashblocks.block-time.
• Builder args: Pass --flashblocks.fixed, --flashblocks.block-time, and --rollup.chain-block-time to the builder binary.
• Rollup config: GetRollupConfig now accepts blockTimeSec instead of hardcoding BlockTime: 1.
• Sequencer benchmark: Sleep duration uses params.BlockTime instead of hardcoded 1000ms.
• OP program benchmark: Passes blockTimeSec from params to NewOPProgramBenchmark.
• CLI defaults: --root-dir defaults to ./data-dir and --output-dir defaults to ./output (no longer required flags).

Testing

• go build ./... passes
• go test ./... passes (matrix_test.go updated to use explicit 1 * time.Second)
• Rebased onto main, conflicts resolved

[cb-heimdall]

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