rpn: split evaluate() into focused helpers

Extract evaluate() (~130 lines) into:
- evaluate(): setup and orchestration (20 lines)
- evaluateTokens(): token loop with handled tracking (12 lines)
- dispatchToken(): single token dispatch (58 lines)
- checkVariableName(): variable name detection for assignment (21 lines)
- processResult(): final stack state and result formatting (25 lines)

All helpers under ~50 lines except dispatchToken (58, close).
Behavior preserved - all tests pass.

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