Small safety fixes: RNC header bounds, hand-rule guards, multi-monitor frontend

[matthewdeaves]

Three small, independent fixes — each in its own commit.

1. dernc: bounds-check UnpackM1
It copied the 18-byte RNC header out of the buffer before checking the buffer was that big, so a file smaller than the header caused an out-of-bounds read. Now it returns early (as "not compressed") when the buffer is too small.

2. power_hand: guard invalid dungeon and out-of-range model
thing_pickup_is_blocked_by_hand_rule() indexed dungeon->hand_rules[model] without checking the dungeon was valid or that model was in [0, CREATURE_TYPES_MAX). An invalid dungeon (e.g. an unowned player) or a bad model dereferenced a bad pointer / read out of bounds. Both are now guarded, with a log line for the offending thing.

3. video: pick the display under the cursor for the pre-window frontend
Before a game window exists (splash/frontend), LbGetCurrentDisplayIndex() always returned display 0, so on multi-monitor setups the splash could size for the wrong screen. It now picks the display containing the mouse cursor, falling back to the old default. (Also the monitor a Wayland compositor opens the window on.)

The first two are adapted from @cerwym's fork, the third from @DoubyCz's — credited in the commit messages. I came across them while working on a native macOS build and they looked worth sending on their own.

Disclosure: reviewed and prepared with the help of Claude Code.

[cerwym]

I guess I should be flattered, but please adjust the commit messages to have the original author as a Co-Author, much like Claude is appending;

So, instruct your tool to add 'Co authored by cerwym (1760289+cerwym@users.noreply.github.com);

This will correctly list me and other authors in the blame for git.

[yani]

Number 3 is completely wrong. Keep that LLM poop out of here. The context of other forks doesn't matter as long as it isn't part of the master. We set the correct monitor trough the config variable DISPLAY_SCREEN:

unsigned short current_display_id = display_id; // default to the already set display_id

This PR breaks that functionality and introduces the cursor placement position which nobody wants when starting up a game.