Add bounds checking to runtime stack_pop and stack_peek

[tcrypt25519]

Both stack_pop and stack_peek in crates/jet_runtime/src/exec.rs have commented-out bounds guards. Currently an underflow panics (or wraps in release) rather than returning a safe error.

• exec.rs:141 — stack_pop: should return a second value (bool / Option / Result) indicating whether the pop was in-bounds.
• exec.rs:152 — stack_peek: should return an error or None when peek_idx >= stack_ptr.