Bounded seeded mocking

Use schema.mockSeeded(seed: 42) for deterministic generation with a validation guarantee. Every returned value passes the schema; the generator validates both the candidate and returned value. Repeatability is for the same schema, limits, seed and Dart SDK, not a promise of identical random sequences across SDK releases.

final schema = object({
  'name': string().min(2).max(12),
  'age': integer().gte(18).lte(65),
});
final value = schema.mockSeeded(seed: 42);
assert(schema.tryParse(value).success);

The supported subset is explicit:

• Standard string schemas with min/max/exact length, required/nonempty, email, and exact-value checks; numeric int, double, and num schemas with gte, lte, and exact-value checks; booleans and their true/false checks.
• Primitive string/numeric/boolean literals, and nullable primitive schemas.
• Ordinary dynamic-valued object schemas with supported children and no cross-field callbacks; tuples with supported children.
• Lists of string, integer, double, num, bool, dynamic, or dynamic-valued maps. List length/min/max/unique checks are supported for string, int and dynamic element types. Other operations or generic combinations fail explicitly.

Numeric sampling is restricted to [-1000, 1000] intersected with schema bounds. Email sampling uses deterministic u<number>@example.com addresses. This is a bounded candidate generator, not a general constraint solver: a satisfiable schema can lie outside its search domain.

Defaults are still handled by validation; they are not assumed to be valid. Dates, unions, templates, pipelines, instance schemas, lazy recursion, custom subclasses, arbitrary refinements, transformations, async checks and cross-field callbacks are outside this contract. Opaque operations are rejected before execution, so a callback that never returns is not invoked by the generator.

Defaults are 64 attempts, depth 16, collection length 256, and 4096 visited nodes per schema inspection or candidate. Override them using maxAttempts, maxDepth, maxCollectionLength, and maxNodes. Contradictions, unsupported features, exhausted search and resource limits throw AcanthisMockException with a reason; no invalid value is silently returned.

Legacy mock([seed]) remains a compatibility, best-effort API and does not gain this validation guarantee. Its list generator now has a finite attempt cap, samples repeated values when uniqueness is not requested, and reports impossible or unsuccessful generation rather than looping forever on a finite domain.