v3.2.0 — Conditional navigation fixes

Conditional navigation was broken for most answer types. This release fixes the
expression evaluators, a regression shipped in 3.1, and the silent loss of date
answers.

Fixed

• Numeric conditions never matched. The operand was compared as a string
  against a number, so = 5 on a rating, slider, NPS or OTP answer was always
  false and != 5 always true. Comparison is numeric, and <, <=, > and
  >= are supported.
• Decimal answers matched every condition. Dispatch tested is int, so a
  slider's double fell through to the catch-all evaluator.
• Boolean and other answers matched every condition. The catch-all
  evaluator returned true unconditionally, so a boolean question always took
  its first branch and could never reach its "no" path.
• NOT_IN meant "not all of these". NOT_IN a,b matched a selection
  containing a, because it negated "every one is present" rather than "any
  one is present". It also compared Options objects against strings, so on a
  multiple-choice answer it matched whatever was selected.
• Operands containing spaces were truncated. Conditions were split on every
  space, so = New York compared against "New" and never matched.
• < DAY(01-01-2025) threw. The two-term date form parsed the operator as
  a date expression and raised a RangeError before comparing anything. A bare
  date operand — < 01-01-2025 — threw for the same reason.
• Date answers were dropped from results. generateResult only recorded a
  DateTime when the step carried a DateResultType, so with any other
  validator — or none — the answer was missing from exportAsJson, from saved
  drafts, and from the map handed to onFinish. Dates are now always recorded,
  formatted when the step says how and ISO-8601 otherwise.
• Container steps disposed their children twice. Introduced in 3.1: nested
  and repeat steps build child views into the tree, so the framework disposes
  each as it unmounts, but they also cascaded from their own dispose(). Any
  form containing a NestedStep or RepeatStep asserted with "A
  TextEditingController was used after being disposed" when it was torn down.
• Restoring a draft threw on a repeat group. A JSON round-trip widens the
  element type, which the unchecked cast on resume rejected. Rating and NPS
  inputs likewise assumed int where a restored draft can hold a double or a
  numeric string.

Changed

• Unknown operators now raise ArgumentError consistently across every answer
  type, rather than silently matching.

Added

• test/unit/expression_test.dart covers every operator against every answer
  type; each case was checked against the behaviour before the fix.
• test/integration/draft_restore_test.dart round-trips a form through JSON
  and rebuilds each restored step, which is what surfaced the container
  double-dispose.