api-modernization: replace signal() usage with sigaction()

[somethingwithproof]

Summary

Replace legacy signal() handlers with sigaction() for consistent, explicit behavior.

Work

• Migrate signal handler setup/reset paths in error.c to sigaction().
• Preserve current intent for fatal-signal handling.
• Define and document handler flags/mask behavior.

Acceptance Criteria

• No runtime signal() registration in production paths.
• Handler install/uninstall logic is deterministic across POSIX targets.
• Behavior matches current operational expectations (including crash-path logging).