fix: track backtrace register availability

[swananan]

Summary

• track RBP availability separately from its recovered value across inline and tail-call unwinding
• reject unavailable CFA and return-address register sources before issuing memory probes
• add regression coverage for a valid zero-valued RBP used as the return-address register

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p ghostscope-protocol -p ghostscope-compiler --lib
• targeted host e2e for zero-valued RBP, undefined RBP propagation, and unavailable return-address recovery
• full host e2e reached backtrace_execution with 31/31 passing, then stopped at the unrelated existing complex_types_execution::test_cross_type_comparisons_local pointer ordered-comparison expectation

Container-topology e2e was intentionally skipped because this change does not affect container, Docker, or PID-namespace behavior.