Define string runtime semantics

[itsfuad]

Goal

Finish string behavior beyond type layout.

Scope

• Decide whether string literals default to string or cstr under expected type.
• Keep string immutable and non-owning.
• Defer owned/mutable text to a separate type if needed.
• Avoid hidden deep clone unless explicitly chosen as language rule.

Validation

• Typechecker/backend tests for string literal expected type.
• Runnable smoke for passing string.