perf: optimise clb core logic

[sidnz]

• Pre-compute variantKeys and compoundConditions at schema-creation time (avoids redundant work on every builder call)
• Removed isBoolean helper - inlined into toStringIfBoolean
• isSimpleSubset uses Object.keys instead of Object.entries
• Early return guard uses !x.length over x.length === 0
• Just optimisations - all 25 existing tests pass