# 0162 — `#run` returning an optional aggregate fails the comptime VM reg→value bridge

## Symptom

A `#run` (or comptime const init) whose function returns an OPTIONAL value
(`?T`, `?i64`, any optional) fails comptime evaluation with:

`error: comptime init of 'X' failed: reg→value: aggregate shape not bridged yet`

A non-optional return of the same type works. This is a pre-existing limitation
in the comptime VM's register→value bridge for optional-typed results; it is
orthogonal to issue 0160 (it reproduces for a value-init optional with no struct
literal anywhere, and for a scalar optional `?i64`).

## Reproduction

```sx
#import "modules/std.sx";
T :: struct { a: i64 = 0; }
mk  :: () -> ?T   { t : T = .{ a = 7 }; return t; }
mk2 :: () -> ?i64 { return 5; }

X :: #run mk();    // error: reg→value: aggregate shape not bridged yet
Y :: #run mk2();   // same class of failure
main :: () { print("ok\n"); }
```
Baseline that WORKS: `Z :: #run (() -> T { return .{ a = 7 }; })();` (non-optional).

## Investigation prompt

`src/ir/comptime_vm.zig` — the reg→value bridge (search "aggregate shape not
bridged" / `regToValue`) handles scalars/structs/slices but bails on an
OPTIONAL-typed result. An optional is `{payload, has_value}` (or a pointer for
`?*T` / a sentinel for `?Closure`); the bridge needs to read the has_value flag
and, when set, bridge the payload as its child type (recursively), producing a
`Value` optional — and a null optional when clear. Add the `.optional` arm to
the reg→value bridge (mirror the value→reg direction, which already builds
optionals — see `makeStringList`/`writeField` optional handling). Verify with
the repro (expect `X`/`Y` to evaluate, `main` prints ok). Add a
`examples/comptime/06xx-...` regression.