- Introduce `Type::Never` (bottom type) to represent unreachable code paths;
  `subtract()` on a union returns `Never` when all members are eliminated
- Add `TypeError::UnreachableCode` reported when a branch is provably dead:
  - then-branch: concrete variable type doesn't match the predicate
  - else-branch: all union members are exhausted by the predicate
- Fix Union-Union unification to use symmetric all-member matching,
  preventing spurious "type mismatch" errors when both sides carry the
  same union type through separate branches
- Infer `while` loop return type as `Union(body, None)` since the condition
  may be false on the first check; bare `break` contributes `None` as well
- Skip `SymbolKind::Parameter` in `detect_dead_then_branch` — parameters
  are polymorphic so predicate checks on them must not be flagged as dead
- Add comprehensive tests for dead code detection, parameter polymorphism
  (no false positives on `contains`/`map`/`flat_map`-style functions), and
  `skip_while`/`take_while` runtime behavior

… new constraints and test cases