Make Mon.syncIdentity's shiny recompute monotonic

syncIdentity unconditionally recomputed mon.shiny from the mon's DVs,
overwriting whatever was there. It is wired into refreshStats, which
SummaryMenu.new calls on every menu open, so a forced shiny -- Mon.new's
opts.shiny path, DVs that do not themselves read as shiny -- got
un-shinied the moment the summary screen opened, even though opts.shiny
already wins over shiny.roll at construction for exactly this case (a
scripted shiny is the cart overriding the roll, not a roll to be
hooked).

mon.shiny now only ever gets PROMOTED by the DV check, never demoted:
`mon.shiny or Mon.isShiny(...)`. A naturally shiny mon and a plain one
are unaffected -- the DV check still runs and still decides the first
time -- and a mon whose DVs are edited to justify shininess later still
promotes normally; only an already-true shiny stops being able to flip
back to false on a later refresh.

src/battle/gen2/Mon.lua

@@ -112,7 +112,13 @@ function Mon.syncIdentity(mon, data)
   if mon.dvs then
     mon.gender = Mon.gender(def, mon.dvs,
       { species = mon.species, level = mon.level })
-    mon.shiny = Mon.isShiny(mon.dvs,
+    -- shiny is monotonic once true, the same as opts.shiny winning over
+    -- shiny.roll at Mon.new: a forced shiny (the scripted-shiny path, DVs
+    -- that do not themselves read as shiny) must not un-shiny the moment
+    -- this runs again, and it runs on every SummaryMenu open via
+    -- refreshStats.  A mon not already shiny still promotes normally if
+    -- its DVs justify it, e.g. after an edit.
+    mon.shiny = mon.shiny or Mon.isShiny(mon.dvs,
       { species = mon.species, def = def, level = mon.level })
     if mon.species == Unown.SPECIES then
       mon.unownLetter = Unown.letterFromDVs(mon.dvs)

tests/engine/gen2_new_seams.lua

@@ -468,6 +468,32 @@ do
     T.eq(forced.shiny, true, "opts.shiny still wins over shiny.roll")
   end)
 
+  -- Mon.syncIdentity (wired into refreshStats, which SummaryMenu.new calls
+  -- on every menu open) used to recompute mon.shiny from DVs unconditionally,
+  -- so opening the summary screen on a forced shiny -- one whose DVs do not
+  -- happen to match the natural pattern -- un-shinied it the moment the menu
+  -- opened.  shiny is monotonic once true: a natural roll or a forced one
+  -- both stay shiny through any later refresh, the way opts.shiny already
+  -- wins at construction.
+  do
+    local forced = Mon.new(DATA, "SEEDMON", 5, { dvs = plainDvs, shiny = true })
+    T.eq(forced.shiny, true, "still shiny straight out of Mon.new")
+    Mon.syncIdentity(forced, DATA)
+    T.eq(forced.shiny, true, "syncIdentity does not clobber a forced shiny")
+    Mon.refreshStats(forced, DATA)
+    T.eq(forced.shiny, true,
+      "refreshStats (SummaryMenu.new's call) does not either")
+
+    -- the natural cases are unaffected: DVs that read shiny stay shiny,
+    -- DVs that do not stay plain
+    local natural = Mon.new(DATA, "SEEDMON", 5, { dvs = shinyDvs })
+    Mon.syncIdentity(natural, DATA)
+    T.eq(natural.shiny, true, "a naturally shiny mon still reads shiny")
+    local plain = Mon.new(DATA, "SEEDMON", 5, { dvs = plainDvs })
+    Mon.syncIdentity(plain, DATA)
+    T.eq(plain.shiny, false, "a plain mon is not promoted to shiny")
+  end
+
   local genderCtx
   withHook("gender.roll", function(nextFn, ctx)
     genderCtx = ctx