chore: set a ceiling for Python SDK customization (#3714)

## Summary

Set a generous 10,000-line ceiling for the Python SDK's remaining
customization of generated files. The current verified patch is 6,558
lines (+6,010 / −548), so normal development has headroom while
unusually large patches need an explicit decision.

This PR intentionally changes only `.castiron-ratchet.json`. Keeping the
policy separate means an SDK change cannot raise its own allowance. A
human approving review is required; the automated technical review is
not policy approval.

Companion tooling PR #3715 adds enforcement using the budget already on
`main`. Land this policy first. This seed alone does not enable a
required check, and it does not change SDK behavior or CODEOWNERS.