JIT: clone expression lowering (object copy)

[PurHur]

Summary

Lower clone $obj to real object copy semantics in JIT/AOT, not stub/null.

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

#817 (closed P1 stub), #145 (constructors/destructors)