Recognize coordinate metadata stored in encoding

[Kkkakania]

Summary

• recognize coordinate criteria such as standard_name, axis, and units when they are stored in a variable's .encoding
• apply the same lookup behavior to standard-name and custom-criteria accessors
• preserve .attrs precedence when the same key appears in both mappings

Closes #49.

Tests

• uv run --frozen --group test pytest cf_xarray/tests -q (288 passed, 1 skipped)
• uvx pre-commit run --all-files