Make default start_from_scratch configurable

[antonchen]

Summary

• read the default threshold from
• apply that runtime default consistently in the library and API index config handling
• add focused tests for the library defaults and the API create-index path

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets --features "openblas" -- -D warnings
• cargo test -p next-plaid --test start_from_scratch_env_test -- --nocapture
• cargo test -p next-plaid-api --test start_from_scratch_env_test -- --nocapture

Implemented entirely by Codex.