[P1] Flow refinement derivation ignores signedness in branch constraints

[ngmachado]

Problem

Branch refinement derivation uses unsigned u256 constant extraction/comparisons regardless of signed integer domain.

Evidence

• Constant extraction into unsigned: src/ast/type_resolver/core/statement.zig:1313
• Unsigned bounds logic over u256: src/ast/type_resolver/core/statement.zig:1320

Impact

Incorrect inferred bounds for signed variables (e.g., conditions involving negatives).

Acceptance Criteria

• Signed and unsigned refinements are derived with correct domain semantics.
• Add signed branch-refinement tests (i8/i16/i256 with negative literals).