feat(signals): activated signal handler delivery

[FlareCoding]

Summary

• Handler-bound signals could only pend: sends never woke a blocked target and no return path built frames, so installed handlers never ran.
• Each syscall return now delivers one pending handled signal (masking per sa_mask, SA_NODEFER, SA_RESETHAND) and a registered rt_sigreturn restores the interrupted context, with blocking waits unwinding as EINTR.
• The sigtest userland app exercises delivery, deferral, unblock-time delivery, and EINTR end-to-end on both architectures.

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