Adds two runtime-settable server parameters to throttle the collection scan phase of background (primary-driven/hybrid) index builds, to reduce foreground cache pressure:

• primaryDrivenIndexBuildScanThrottleBytes (default 0 = disabled)
• primaryDrivenIndexBuildScanThrottleSleepMillis (default 5)

When enabled, the scan loop in _doCollectionScan accumulates bytes inserted into the bulk builder and performs an interruptible sleep each time the byte threshold is reached. Foreground builds are unaffected.

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