v0.9.11: Type system TODO fixes

v0.9.11: Fix type system TODOs — Future trait, array subscript, struct literal type inference

• Future trait checking: extract inner type from Future instead of returning the
  Future type itself (new_resolver.rs:724)
• Array subscript element type: infer from base type (Array, Slice, Vec) instead
  of hardcoded I64 (typecheck.rs:344)
• Remaining struct-related TODOs (1159, 1172, 2217) need structural changes
  (field type storage maps) — deferred to avoid scope creep