Fix ipairs(self) type resolution in generic class methods

When iterating with ipairs(self) inside a generic method where self is
list<T>, the loop variable v was resolving to unknown instead of T.

The fix ensures the V->T resolution chain is preserved through three
layers: sign resolution finds the mapping via class generic map and
extends tables, generic.lua preserves the resolved wrapper when a
generic maps to another generic, and compiler.lua allows these
resolved-to-generic types through the return type filter.

Supports both @field [integer] T and @class list<T>: {[integer]:T}
class definition patterns.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>