Execute drop glue for Drop terminators

[Stevengre]

Summary

interior-mut-fail.rs should pass under rustc, but KMIR failed on the second RefCell::borrow_mut() by reaching panic_already_borrowed(noBody).

The root cause was not RefCell writeback. It was that KMIR still treated MIR Drop terminators as no-ops, and the pre-proof SMIR reduction also discarded drop_in_place::<...> bodies because Drop was not modeled as a reachability edge.

This PR fixes that in two steps:

• preserve reachable drop glue during SMIR reduction by adding Drop-based edges to call_edges
• execute Drop terminators through the normal call path using reachable drop_in_place::<T> bodies

After that, the temporary RefMut guard from the first borrow_mut() is actually dropped, the borrow flag is released, and interior-mut passes.

Commit guide

1. fix(kmir): execute drop glue for drop terminators
   This adds the semantic fix and renames interior-mut-fail.rs to interior-mut.rs, with an intermediate show output proving the case now reaches #EndProgram.
2. test(kmir): drop redundant interior-mut show output
   This removes that temporary show output once it no longer carries diagnostic value.

Validation

• make build
• uv run pytest src/tests/integration/test_integration.py -v --timeout=600 -k 'interior-mut or interior-mut3-fail'
• direct proofs:
  • interior-mut.main passes
  • interior-mut3-fail.main still fails with the existing misaligned-pointer issue

Notes

I also attempted a local make test-integration run after wiring deps/stable-mir-json into the worktree. It started cleanly, but I did not wait for the full suite to complete locally.