Refactor calculator.Parse to make RPN vs percentage parsing boundaries explicit

- Removed parseRPNFallback from strategy registry
- Parse() now only handles percentage calculations (no RPN fallback)
- ParseRPN() is now a separate, explicit function for RPN expressions
- Added clear documentation explaining Parse() only handles percentages
- Removed TestParseRPNFallthrough test (tested implicit RPN fallback)

This makes the codebase easier to understand and maintain by having
clear boundaries between percentage and RPN parsing.

All tests pass. 2 files changed, 1 deletion(-).

0 files changed, 0 insertions, 0 deletions