Improve 64-bit Linux and retail Prey compatibility

[bobmoser76]

Summary:

• fix 64-bit math, script VM, PVS, hashing, networking, thread-handle, and asset-loading assumptions
• rename the staged runtime content directory from basepy to basepr
• improve retail collision, AAS, savegame, particle, renderer, audio, GUI, and gameplay compatibility
• recover players from overlapping collision seams without slow or jittering movement
• fix the Linux shutdown segmentation fault caused by truncated pthread handles

GameLib dependency:

• Improve Prey gameplay and collision compatibility openPREY-game#2

Validation:

• full Meson client, dedicated-server, and unified game-module build completed
• Meson staging install completed
• affected quicksave replay maintained stable ground height and normal walking velocity
• shutdown exits normally under GDB and directly with status 0
• git diff --check passes

Local generated binaries, diagnostic captures, and retail/local content overlays were intentionally excluded.