perf(input): bound spatial-index memory and query work

[devmobasa]

Summary

• cap per-shape and aggregate spatial-index memberships with conservative global fallback
• defer cell-key materialization until coverage is admitted
• extract grid arithmetic and storage behind a focused module boundary
• bound extreme-radius queries and reject invalid hit-test tolerances
• preserve candidate completeness across every fallback path

Validation

• focused spatial-grid, hit-test, input-state, and config-validation tests
• the same focused tests with --no-default-features
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• git diff --check