Clarify R503: require terminal returns only when another branch returns a value

[coderabbitai]

Summary

Clarify .rules/python-return.md so R503 does not require an explicit bare return in functions whose only possible result is None.

Rationale

R503 currently says “Don't rely on implicit None—always return something at the end.” This conflicts with the narrower intent expressed by R502 and led to an invalid request to add return to an all-None helper.

State the rule as:

Don't rely on implicit None if the function may return a value elsewhere—always return something at the end.

This retains the safeguard for mixed value/None control-flow while avoiding redundant bare returns for procedures.

Affected areas

• .rules/python-return.md
• The R503 heading, explanation, and BAD/GOOD examples
• Alignment between R502 and R503

Acceptance criteria

• Update R503 to scope the terminal-return requirement to functions that return a value in another branch or path.
• Preserve R501’s preference for bare return over return None.
• Make the R502 and R503 prose/examples internally consistent.
• Do not require an explicit bare return for functions whose only result is None.

Backlinks

• Raised by @leynos during leynos/lading#164.
• Discussion: Remove backwards-compat shims guarding private symbols (#163) lading#164