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GridCraft keeps workbook data in hidden defined names so it travels inside the xlsx file. The existing items write one name holding one string literal, which past Excel's 255-character literal and 8,192-character formula limits gives a file Excel does not accept. cmd/stash.rs stores an item `key` as hidden workbook-scope names _gc_<key>_0, _gc_<key>_1, ..., each a formula of string literals of at most 255 characters (UTF-16 units, as Excel counts them) joined with &, at most 8,192 characters per name. Reading concatenates the chunks in order and stops at a gap; it also accepts the older single _sc_<key> name. Names are matched case-insensitively and only at workbook scope. Keys are limited to letters, digits, _ and ., items to 4,194,304 characters, and junk around or inside the literals, duplicate or out-of-order chunks, non-JSON and over-deep JSON all read safely. keys(prefix) lists the stored items that start with a prefix. Custom views, scripts, the watch list and scenarios keep their current storage; nothing reads or writes the new names yet. docs/architecture.md describes the engine building blocks added in this series: undo groups, the recording buffer, document trust and this storage.