feat: measure incremental authoring plans

[snkmcb]

Summary

• add an explicit --authoring incremental benchmark mode for the M5 bounded authoring path
• feed bounded reader batches directly into FeaturePlanBuilder without retaining feature or plan vectors
• identify the authoring mode in CSV output and add a focused CMake smoke test
• document the measurement procedure in the scalability baseline

Validation

• cmake --build build/m5 --target usd-vector-benchmark
• incremental smoke: 2 features, batch_count=1, max_batch_features=1, retained_feature_bytes=0
• 10,000-point measurement: batch_count=40, max_batch_features=256, retained_feature_bytes=0
• git diff --check

CMake Tools could not run CTest because no configure preset was active; the rebuilt executable smoke checks passed.