Replace the per-record OplogEntry::parse and the duplicated repl::ApplyOps::extractOperationsTo traversals in aggregateSizeCountDeltasInOplog with a private BSON-view path that reads only the fields we actually consume.

This keeps the existing OpSizeCountExtractable concept and extractSizeCountDeltaForOpImpl template intact for the typed-input call sites. Two extractors live side by side, with a clear split: the BSON-view path drives the cursor scan, the typed template path drives op-observer / oplog-applier callers that already hold parsed objects.

Correctness checklist:

• Tenant inheritance preserved: applyOps with tid only at the outer level produces the same NamespaceString for inner ops .
• m array variant on commitTransaction continues to contribute no delta (rejected by the type check on m).
• m inheritance from outer to inner op (when inner lacks m) matches today's behavior.
• kCreate / kDrop fast-count-store detection produces the same result as CommandHelpers::parseNsCollectionRequired for valid input.
• lastTimestamp is advanced for the same set of entries as today.
• All existing log IDs and severity levels preserved and massert IDs and messages preserved.

Acceptance criteria:

• Public interface of aggregateSizeCountDeltasInOplog unchanged.
• No behavior change in deltas, advanced timestamps, or emitted metrics.
• Existing replicated_fast_count_test.cpp and replicated_fast_count_metrics_test.cpp pass unchanged.
• New focused test for the newly added code.
• Benchmark from the parent's sub-task shows a measurable improvement on the scan workloads, attached to the PR description as a delta against the master baseline.
• ReplOperation aggregation path is untouched and shows no regression.