Language: foreach by-reference ($k => &$v)

[PurHur]

Summary

Implement foreach loops that assign by reference, including nested references and array modification during iteration.

Context

• See docs/unsupported-syntax.md and docs/capabilities-syntax.md.
• North Star 2 self-host: #1056.

Acceptance criteria

• Parser + CFG lowering for the construct
• VM semantics with regression tests
• LLVM JIT lowering (or explicit stub policy in SelfHostBuiltinPolicy)
• Bootstrap fixture if required for self-host probe
• Documentation updated

Related issues

#140 (references), #66 (arrays)