Add runtime string construction and concatenation

[itsfuad]

Goal

Add runtime construction and concatenation for owned str values.

Parent

Follow-up to #29. Existing literals, borrowed views, ranges, and compiler operations remain canonical.

Required semantics

• Construct immutable owned str values from runtime data.
• Define concatenation result ownership and allocator provenance.
• Preserve UTF-8 validity and byte-length semantics.
• Reuse existing owned string carrier and allocator descriptor.
• Keep construction separate from C and FFI boundary conversion tracked by the sibling follow-up.

Constraints

• No mutable string contents.
• No hidden process allocator when an input owner carries allocator provenance.
• No alternate string carrier or backend-only shortcut.
• Source operations must lower through canonical semantic, HIR, MIR, and LLVM evidence.

Validation

• Typechecker and ownership tests for construction and concatenation.
• HIR, MIR, and LLVM tests for allocator and cleanup behavior.
• Positive runtime fixtures for empty, ASCII, UTF-8, chained, and temporary values.
• Negative fixtures for invalid ownership or unsupported operand forms.
• 32-bit and 64-bit carrier checks.