Module conditions

Condition-expression model and triple-array serialization for the policy builder's condition editor (e.g. if/branch nodes and fault-injection vars).

This is the TypeScript counterpart of the Rust condition parser in src/vars/mod.rs (Expr::parse): the triple-array dialect where a rule is [subject, op, value], the negated [subject, "!", op, value], or a unary [subject, "present"|"absent"|"is_null"]; groups are ["AND"|"OR", ...] or ["NOT", child] (heads case-insensitive). Subjects are either flat var names (http_*, arg_*, cookie_*, or a bare var name) or JSONPath queries over a request/response JSON body ($..., request_body:$..., response_body:$...).

ConditionNode is the builder-friendly model the UI edits; toExpr/fromExpr convert it to/from the triple-array form used in policy YAML. toVarsList/fromVarsList handle the fault-injection-style vars list, which is an OR-of-ANDed-expressions rather than a single top-level AND.

Round-trip fidelity: fromExpr(toExpr(m)) deep-equals m for any model built by this module's own constructors, and toExpr(fromExpr(e)) equals e for any expression this module can represent. Anything the builder model cannot represent (non-scalar operands, malformed groups, ...) makes fromExpr/fromVarsList return null rather than a mangled model. Two narrow, deliberate exceptions to byte-identical round-tripping: != is normalized to its ~= alias at parse time (identical Rust-side semantics), and a var-subject name typed with a legacy $/${...} wrapper (the VarInput catalog-suggestion convention) has that wrapper stripped at serialize time (see stripLegacyDollar) -- neither changes what the expression means.

Pure module: no React imports, no side effects.