winrl: validate coordinates and message window access

[j-lojek]

Summary

• add try_screen_offset() helper to validate x,y before converting to size_t map offsets
• remove modulo-based indexing in RL screen writers and early-return on out-of-range coordinates
• guard in_yn_function message extraction against missing/empty message windows
• ensure copied message buffers are explicitly NUL-terminated

Bugs fixed

• S4: signed-to-size_t index path in RL screen writers
• S5: unsafe .back() on potentially empty win->strings

Validation

• code-level validation and patch review of all affected call paths
• full target build is currently blocked on GCC15 compatibility issue tracked in PR 10