fix(backtrace): quit from recursive edits

[kiennq]

Issue

Backtrace buffers enter a recursive edit. Invoking top-level to quit them is currently caught by every nested command loop, so the recursive edit restarts instead of unwinding. This leaves the backtrace buffer effectively unquittable and prevents surrounding cleanup from running.

Solution

• Catch top-level only in the outermost command loop.
• Let nested recursive edits propagate top-level so their unwind cleanup runs.
• Add a regression covering nested recursive-edit propagation and cleanup.

Verification

• git diff --check upstream/main...HEAD passes.
• The regression is included in neovm-core/src/emacs_core/eval_test.rs.
• Windows neovm-core test compilation is currently blocked on unmodified upstream/main by the separate Unix-signal issue addressed in fix(windows): gate Unix signal handlers #276.