load-save: fix for the current title screen interface

[Alistair-Afton]

Summary

load-save still drove the pre-Steam title screen interface: it wrote sel_menu_line (the field is now selected), fed SELECT, and then looked for a standalone viewscreen_loadgamest save picker that no longer exists. The script was effectively dead.

The current viewscreen_titlest manages the whole continue-game flow through its mode field (main menu -> active world list -> save list) and only responds to mouse input. The script now runs in a gui.script coroutine, clicks list rows at their pixel positions, and follows the mode transitions:

• mode 0 (main menu): clicks "Continue" to reach the world list
• mode 2 (world list): enters each world in turn and searches its save list (mode 3), backing out with LEAVESCREEN between misses
• mode 3 (save list): also handled as a starting point
• verifies the save being loaded via the load screen's cur_save header instead of trusting the row click, so a missed probe errors out instead of silently loading the wrong save

Related: DFHack/dfhack#978 (enhancement wishlist; this restores basic functionality). This also unblocks the TODO in test.lua's ensure_fortress that wants to load a specified save dir via load-save.

Testing

• load-save region1 from the main menu: enters world list -> save list -> loads region1, verified via world.cur_savegame.save_dir
• starting while the world list or save list is already open works as well
• load-save bogus: reports "Can't find save: bogus" and leaves the screen on the world list
• running with a fortress already loaded reports "Can't find title or load game screen"