Reuse step buffer in SobolBrownianBridge random sequence generators

[PureSaber]

Summary

• Avoid heap allocation of a factors-sized vector on every nextSequence() call in SobolBrownianBridgeRsg and Burley2020SobolBrownianBridgeRsg.
• Reuse mutable stepBuffer_ resized once per path.

Test plan

• CI (existing Sobol/Brownian-bridge MC tests; sequences unchanged)

Made with Cursor

[CLAassistant]

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