chore: add import stats observability and clang-format

[snkmcb]

Summary

• extend the shared Gaussian import statistics record with coordinate conversion, warning counts, and filter rejection counts
• wire the statistics through PLY, SPZ, and SOG decoders and file-format layers
• add repository clang-format policy, changed-line checks, contributor commands, and PR CI
• ignore the OpenStrata-generated strata.lock

Verification

• ost build --target cy2026 --profile usd --compiler host
• ost test --target cy2026 --profile usd (13/13 passed)
• python scripts/check_clang_format.py --changed (12 files checked)
• git diff --check