feat: add COPC runtime streaming research probe

[snkmcb]

Summary

• add an opt-in COPC runtime-streaming probe behind USDGEO_BUILD_BENCHMARKS
• select deterministic point-data tiles by hierarchy level and decode only those ranges
• report hierarchy size, selected tiles, decoded points, selected range bytes, and source bytes read as TSV
• document the experiment and keep host LOD, OpenUSD payload composition, and renderer loading as separate research
• align converter usage text with documented .copc.laz support

Validation

• get_errors: no errors in the new C++ and CMake files
• git diff --check: passed
• isolated CMake configure with USDGEO_BUILD_USD=OFF, USDGEO_BUILD_TESTS=OFF, and USDGEO_BUILD_BENCHMARKS=ON: passed
• full compile was unavailable because CMake Tools had no registered build target in the current VS Code session