docs: describe Thievery chest lock minigames and seized pins

[ryanbarlow97]

Summary

Documents the chest lockpicking changes in TF-Minecraft/Thievery#33.

• SYSTEM.md
  • The chest row of the responsibilities table.
  • A "Chest lock minigames" section covering:
    • the pin grid dialog: phases, cell textures, Give up, clicks while the dialog redraws, and the sprite label workaround;
    • the floating lockpick ring: display entities, keybind keys, holding the thief still and crash recovery, passes, lag compensation and placement;
    • failures and interruptions shared by both games.
  • A "Chest probe: seized pins" section: placement, counts, marking, breaks and the pin-count formula.
• TEST_MATRIX.md: R4 now reads "unchanged once solved", and new checklists cover the grid (G1-G7), the ring (D1-D15) and seized pins (G13-G18).

Merge this alongside Thievery#33.

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