v0.34.0

No schema change: x-schema-version stays 1.18. New API: the engine evaluates
parametric exercises (resolveExerciseVariables, evaluateExpression). No
new rule, so nothing a consumer validates changes.

The engine evaluates parametric exercises (engine#220)

resolveExerciseVariables(exercise, { random, values }) turns a parametric
exercise (schema 1.14) into a concrete instance: it samples every sampled
variable with the given random source, evaluates every computed one in
declaration order, rounds each to its display precision and substitutes every
reference to a declared variable in the exercise's string fields. It returns
the exercise, the values (to persist and replay) and the tolerance of every
accepted answer that is one reference to a variable with tolerance.
evaluateExpression(expression, values) is exported on its own. See
variables.

Why: the engine validated the contract and left evaluation to the consumer,
so the reference app carried a second parser for the same expression
language (adaptive-learner#3109). The engine now evaluates on the parser that
validates: one grammar, one implementation. The contract is the app's, so it
can switch without changing its call sites; its own test cases are ported
unchanged and pass. Two differences follow from the single grammar: an
expression the validator rejects (.5, a unary +) does not evaluate either,
and a reference with spaces ({{ a }}), which the validator reads as a, is
substituted (the app left it literal). The grade lookup for evaluation
follows with its first consumer.