Add iterable for loops

[itsfuad]

Goal

Add iterable for loops after existing conditional and infinite-loop foundation.

Already delivered

• Conditional for condition { ... } parsing, checking, CFG, MIR, and backend lowering.
• Infinite for { ... } form.
• Break and continue control-flow support.

Remaining scope

• Add direct iteration over fixed arrays, dynamic arrays, and slice views.
• Define binding forms for value-only and index-plus-value iteration.
• Preserve copy-versus-borrow rules for move-only elements; do not extract move-only elements by indexed value access.
• Define iterator protocol only after optional narrowing in Add optional narrowing and payload access #28 makes a next -> ?T contract usable.
• Avoid dynamic dispatch for statically known arrays and slices.
• Allow dynamic dispatch only when source is already interface or dynamic.

Dependencies

• Arrays and slices foundation: Implement array and slice value semantics #27, complete.
• Optional narrowing and payload access: Add optional narrowing and payload access #28, required before generic iterator protocol.
• Optional niche expansion Add optional niche layouts #30 is not required.

Validation

• Parser, typechecker, ownership, HIR, CFG, MIR, and backend tests.
• Positive x_test fixtures for fixed arrays, dynamic arrays, slice views, index-value bindings, and infinite loops.
• Negative fixtures for invalid sources, mutation conflicts, and move-only extraction.
• Runtime cleanup tests for break, continue, return, and temporary iterable owners.