Translate the PC's CHANGE BOX save flow (PcMenu.lua)

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.

tests/engine/gen2_save_menu_translation_test.lua

@@ -147,21 +147,26 @@ do
   T.check(not Strings.active(), "the catalog is unloaded for the checks after this one")
 end
 
--- src/ui/gen2/PcMenu.lua:savePrompt() returns SaveMenu.OVERWRITE_PROMPT/
--- SAVING_PROMPT straight through to its own `lines[1]`/`lines[2]`
--- Chrome.print calls (the PC's CHANGE BOX save uses the same two prompts).
--- Indexing a plain string with [1]/[2] returns nil, not characters, so this
--- shape is a cross-file contract: it caught a real regression during review,
--- where routing these through a single Strings.source()-wrapped string (to
--- translate SaveMenu's own screen) silently turned them into non-table
--- values and left PcMenu's overwrite/saving prompt blank.
+-- src/ui/gen2/PcMenu.lua:savePrompt() shares SaveMenu's overwrite/saving
+-- prompts through SaveMenu.OVERWRITE_PROMPT_SOURCE/SAVING_PROMPT_SOURCE and
+-- SaveMenu.twoLines(), rather than duplicating them -- both exported below,
+-- both used by PcMenu's own translation test
+-- (tests/engine/gen2_pcmenu_changebox_save_translation_test.lua). Checked
+-- here that they stay callable the shape twoLines() expects: a table in,
+-- one \n-joined string out with the split back on load.
 do
-  T.eq(type(SaveMenu.OVERWRITE_PROMPT), "table", "OVERWRITE_PROMPT stays a table for PcMenu.lua")
-  T.eq(SaveMenu.OVERWRITE_PROMPT[1], "There is already a", "and its first line stays indexable")
-  T.eq(SaveMenu.OVERWRITE_PROMPT[2], "save file. Is it", "and its second line")
-  T.eq(type(SaveMenu.SAVING_PROMPT), "table", "SAVING_PROMPT stays a table for PcMenu.lua")
-  T.eq(SaveMenu.SAVING_PROMPT[1], "SAVING… DON'T TURN", "and its first line stays indexable")
-  T.eq(SaveMenu.SAVING_PROMPT[2], "OFF THE POWER.", "and its second line")
+  T.eq(SaveMenu.OVERWRITE_PROMPT_SOURCE, "There is already a\nsave file. Is it",
+    "OVERWRITE_PROMPT_SOURCE stays the cart's own \\n-joined text")
+  T.eq(SaveMenu.twoLines(Strings(SaveMenu.OVERWRITE_PROMPT_SOURCE))[1], "There is already a",
+    "and twoLines() splits its untranslated fallback back to the first line")
+  T.eq(SaveMenu.twoLines(Strings(SaveMenu.OVERWRITE_PROMPT_SOURCE))[2], "save file. Is it",
+    "and its second line")
+  T.eq(SaveMenu.SAVING_PROMPT_SOURCE, "SAVING… DON'T TURN\nOFF THE POWER.",
+    "SAVING_PROMPT_SOURCE stays the cart's own \\n-joined text")
+  T.eq(SaveMenu.twoLines(Strings(SaveMenu.SAVING_PROMPT_SOURCE))[1], "SAVING… DON'T TURN",
+    "and twoLines() splits its untranslated fallback back to the first line")
+  T.eq(SaveMenu.twoLines(Strings(SaveMenu.SAVING_PROMPT_SOURCE))[2], "OFF THE POWER.",
+    "and its second line")
 end
 
 -- A translation with a THIRD line (a second embedded "\n") has nowhere on