side-effecting operations like `buf.free` within a reduce will double-free

We need `buf.free` for efficiency, but currently one can smuggle a buffer into a closure's environment and then free it.
Inside a

Some ideas:

- `defer` requires something like `Copy` in Rust
- Compiler builds in a check for "known bad" values that shouldn't be allowed in a closure's environment (hack alert)

Start by tracing how buf.free is handled inside reduce and how a closure captures its environment. Reproduce the double-free case described in the issue, then determine the ownership or compiler rule needed to prevent side-effecting operations from escaping safely; the issue's proposed approaches are defer with Copy-like behavior or a compiler check for known-bad captured values.

Written by the indexing model from the issue text.