feat(core): add arithmetic functions ADD/SUBTRACT/MULTIPLY/DIVIDE/NEGATE - #193

Summary

Implements #192: row-level arithmetic as prefix functions, consistent with the existing ABS/SQRT/MOD design. No grammar precedence machinery is introduced — the new functions slot into the existing functionCall → Function → *FunctionMapper pipeline.

Arity rationale: ADD/MULTIPLY are associative and commutative, so they map 1:1 to the natively-variadic $add/$multiply (and to a JPA fold, exactly like CONCAT today). SUBTRACT/DIVIDE are only left-associative and their backends are binary-only, so they stay strictly binary; chains are expressed by nesting: ADD(MULTIPLY(price, 1.22), fee).

Examples

select multiply(price, 1.22) as gross
where subtract(qty, reserved) > 0
select multiply(sum(price), 2)

Changes

• Function enum — ADD(-1), SUBTRACT(2), MULTIPLY(-1), DIVIDE(2), NEGATE(1) in ARITHMETIC; getMinimumArguments() returns 2 for the variadic arithmetic functions
• Lexer/Parser — five new function-name tokens; functionArgs already supports variadic args, nested calls and numeric literals, so no other grammar change
• JpaFunctionMapper — cb.sum/diff/prod/quot/neg; add/multiply fold like concat
• MongodbFunctionMapper — $add/$subtract/$multiply/$divide; negate as $multiply [-1, x]
• Querity factory — add(...), subtract(a, b), multiply(...), divide(a, b), negate(a)
• Elasticsearch — unsupported (throws UnsupportedOperationException), consistent with every other function

Notes

• Integer division: DIVIDE maps to cb.quot, which follows the backend's integer-division semantics for integer operands; documented in the enum Javadoc. Happy to coerce to decimal instead if preferred.
• Backward compatible: the new names are not valid anywhere in today's grammar.

Testing

• Unit tests: enum arity/category, FunctionCall arity validation, parser (variadic, nested, decimal literals, case-insensitive), JPA mapper (incl. fold verification), MongoDB mapper (document shapes), factory helpers, Elasticsearch unsupported
• Integration tests in QuerityGenericTestSuite (ADD/SUBTRACT/MULTIPLY in filters) — green on H2, PostgreSQL (Testcontainers) and MongoDB
• ./mvnw verify — JaCoCo per-class coverage checks met on all touched modules

Closes #192

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