✨ Refactor: box captured variables at declaration (Option B upvalue design)

[timfennis]

Background

The VM currently uses a Lua-style open/closed upvalue system. Captured variables start as stack slots and migrate to heap-allocated cells when their enclosing frame exits. This works correctly but carries ongoing complexity: open_upvalues, capture_upvalue, close_upvalues, CloseUpvalue opcode, and the UpvalueCell::Open/Closed distinction.

Bug B5 (materialize_upvalues_in_args) was a direct consequence of this design and was fixed by deleting the offending function (see #discussion in UPVALUE_DESIGN.md). However, the underlying open/closed mechanism remains and could produce similar bugs in future edge cases.

Proposed change

Mark captured variables during the analysis/compilation pass. Instead of starting life as stack slots and migrating to the heap on frame exit, captured variables are heap-allocated (Rc<RefCell<Value>>) from the moment of declaration. The outer frame and all inner closures hold a reference to the same heap cell from the start — no open→closed transition.

Non-captured locals are unaffected and remain cheap stack slots.

What changes

• The analyser already tracks which variables are captured by inner closures — that information is used to tag declarations
• The compiler emits GetUpvalue/SetUpvalue for captured variables in the outer function too, not just in inner closures
• UpvalueCell::Open variant is deleted — cells always hold a Value
• open_upvalues, capture_upvalue, close_upvalues, and CloseUpvalue are all removed
• Loop iteration isolation (currently CloseUpvalue) is replaced by the compiler emitting "allocate fresh cell, copy current value" at the top of each loop body for captured loop variables

Trade-offs

	Option B (this issue)
Lines changed	~200–400, multiple files
Risk	Medium
Eliminates open/closed bug class	Yes
Runtime cost	Heap deref for captured vars (small fraction of all locals)

References

See ndc_vm/UPVALUE_DESIGN.md for the full analysis including the alternative options that were considered.