v5 Semantics: import/export resolution

New checks

• module names are unique
• main exists and is last
• imports refer to earlier modules
• import aliases are unique in the module scope
• exported names exist and are unique
• member access uses an imported module alias
• member names must be exported

Member access rules

Given m.add:

1. m must be a module alias
2. add must be in that module's exports

This keeps module boundaries explicit and readable.