fix: restore default crash signal handling

[camellia-nexus]

Addresses CRASH-001 (Protocol half).

• remove the async-signal-unsafe SIGSEGV handler and global callback
• preserve the operating system default non-zero crash semantics
• remove the now-unused direct backtrace dependency
• add an isolated Linux release child-process regression for signal status, callback absence, and config immutability

Coordinated Client change will remove the sole registration call.

Local validation:

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo clippy --locked --release --test crash_signal -- -D warnings
• cargo test --locked --all-targets --all-features (103 passed)
• release crash_signal regression passed
• cargo audit --deny warnings (338 dependencies)