Implement array and slice value semantics

[itsfuad]

Goal

Make [N]T and []T useful beyond type/layout support.

Scope

• Add array literals only after syntax is locked.
• Add indexing through parser/typechecker/HIR/MIR/backend.
• Support element mutation through explicit pointer or mutable-container path.
• Decide bounds behavior before backend lowering.

Validation

• Parser/typechecker tests for literals/indexing.
• MIR/backend lowering tests.
• Runnable x_test smoke for read and mutation cases.