feat: implement mapping evaluator with built-in functions

[alvinreal]

Summary

Closes #17 — Mapping evaluator.
Part of #3 (Mapping DSL epic).

Changes

Evaluator (src/mapping/eval.rs):

• All 6 statement types: rename, select, drop, set, default, cast
• Path resolution: nested fields, array indices (positive & negative), wildcards
• Expression evaluation with precedence: arithmetic, comparison, logical, unary
• String concatenation via + with auto-coercion to string
• Type casting: int↔float↔string↔bool with null handling
• Division/modulo by zero detection
• Mixed int/float arithmetic with automatic promotion
• Truthiness for all types (null=false, empty string=false, 0=false, etc.)

Built-in functions (src/mapping/functions.rs) — 30+ functions:

• String: lower, upper, trim, trim_start, trim_end, len, replace, contains, starts_with, ends_with, substr, concat, split, join, reverse
• Type: to_int, to_float, to_string, to_bool, type_of
• Math: abs, min, max, floor, ceil, round
• Null: is_null, coalesce, default
• Aliases: lowercase/downcase, uppercase/upcase, length/size, ltrim/rtrim, substring, int/float/str/string/bool/typeof

Tests (110+):

• Eval: rename (simple, nested, nonexistent), select (single, multi, nested), drop (single, multi, nested), set (all literal types, path refs, expressions, functions), default (missing, existing, null), cast (all type combinations, null, errors)
• Expressions: arithmetic chain, string concat, comparisons, logical ops, not, mixed int/float
• Functions: all 30+ functions with aliases, error cases (unknown, wrong arg count)
• Multi-statement: rename+select, set+drop, complex 7-statement pipeline
• Edge cases: division by zero, modulo by zero, null input, empty map, array index, truthiness

All checks pass: cargo build, cargo test (575 tests), cargo fmt --check, cargo clippy -D warnings.