PcMenu.lua's own CHANGE BOX save confirmation stayed in English on every
non-English build: its overwrite/saving/done prompts and its YES/NO choice
were bare literals, invisible to a translation mod's `strings` registry,
even though the overwrite/saving prompts are the exact same two cart
messages Gold's SAVE screen (SaveMenu.lua) already routes through
Strings().
SaveMenu.lua now exports OVERWRITE_PROMPT_SOURCE, SAVING_PROMPT_SOURCE and
twoLines() so PcMenu:savePrompt() can call through them instead of
duplicating the two shared prompts. The now-unused plain
OVERWRITE_PROMPT/SAVING_PROMPT tables (PcMenu's old direct-read contract)
are gone along with them, removing a second copy of the same literal text
that had to be kept in sync by hand. The CHANGE BOX confirm prompt itself
("#MON BOX, data\nwill be saved. OK?") gets its own new Strings.source()
key -- it is not shared with SaveMenu, and the port's own English text is
already a same-shape truncation of the cart's three-line
_ChangeBoxSaveText, confirmed against poke-corpus GoldSilver
en_msg.txt:4897. The done message and YES/NO now go through the exact
same Strings() keys SaveMenu.lua already uses ("%s saved\nthe game.",
"Could not save.", "YES", "NO"), so a translation covering SaveMenu's
screen needs no PcMenu-specific fork for any of those.
Added tests/engine/gen2_pcmenu_changebox_save_translation_test.lua,
mirroring gen2_save_menu_translation_test.lua's technique for PcMenu's own
CHANGE BOX flow, and updated the stale cross-file-contract regression case
in gen2_save_menu_translation_test.lua to match the new shared-export
shape instead of the old plain-table one.
bryanthaboi