[codex] expose dtype option

[DGoettlich]

Summary

• Add a dtype keyword to scpc() with literal options float16, float32, and float64.
• Thread the dtype through the exact spatial distance/kernel/basis path where dense spatial arrays dominate memory.
• Add targeted dtype propagation tests and document the new argument in the README.

Validation

• uv run ruff check --fix .
• uv run ruff format .
• uv run ty check
• uv run pytest
• uv run pre-commit run --all-files