Fixes and enhancements for Cashier checkout service

[nelsonmfinda]

Bug fixes:

• Remove double rounding (rules + total) → single rounding point
• Add zero/negative quantity handling in pricing rules
• Validate rule configuration (duplicate codes, threshold, denominator, numerator)

Performance:

• Cache total in session state (O(1) instead of O(n) per total() call)
• Pre-build rule index at session creation

Validation:

• Add price upper bound (£999,999.99)
• Validate product codes (non-empty, non-whitespace)
• Make validate_rules!/1 public for testing

Testing:

• Add doctests for documentation examples
• Add comprehensive Defaults validation tests