Fix Substitute zero-HP boundary

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

src/battle/MoveEffects.lua

@@ -281,11 +281,11 @@ MoveEffects.primary = {
                failed = true }
     end
     local cost = math.floor(user.mon.stats.hp / 4)
-    -- substitute.asm only fails on subtraction underflow (current HP
-    -- strictly below maxHP/4); at equality the substitute is built and
-    -- the user is left standing on exactly 0 HP (it faints only when
-    -- the engine next checks HP, not here)
-    if user.mon.hp < cost then
+    -- A Substitute costs one quarter of max HP, rounded down. Do not let
+    -- the cost consume the user's last HP: the move must fail at the exact
+    -- boundary as well as below it, or the next turn's HP guard can leave a
+    -- trainer battle unable to progress.
+    if user.mon.hp <= cost then
       return { romText(battle.data, "_TooWeakSubstituteText", "Too weak to make\na SUBSTITUTE!"),
                failed = true }
     end

tests/parity_substitute_anim.lua

@@ -75,6 +75,19 @@ do
   check(anyText(tb, "SUBSTITUTE"), "the failure text still prints")
 end
 
+-- Exact quarter HP is also not enough: accepting it would leave the user at
+-- 0 HP with substituteHP set, so the next trainer-battle turn cannot advance.
+do
+  local tb = freshBattle()
+  local cost = math.floor(tb.enemy.mon.stats.hp / 4)
+  tb.enemy.mon.hp = cost
+  tb:performMove(tb.enemy, tb.player, { id = "SUBSTITUTE", pp = 10 }, false)
+  check(tb.enemy.substituteHP == nil and tb.enemy.mon.hp == cost,
+        "exact quarter HP cannot create a zero-HP substitute")
+  check(not anyAnim(tb, "SUBSTITUTE"),
+        "the exact-boundary failure plays no animation")
+end
+
 -- .alreadyHasSubstitute: same, with a doll already standing
 do
   local tb = freshBattle()

tests/run_tests.lua

@@ -446,16 +446,17 @@ check(misted2.stages.attack == nil
       and mistMsgs[1]:find("MIST", 1, true) ~= nil,
       "primary stat drop still blocked by MIST")
 
--- Substitute boundary: built at exactly 1/4 max HP, leaving 0 HP
--- (substitute.asm only fails on subtraction underflow)
+-- Substitute boundary: the move must fail when its quarter-HP cost would
+-- consume all current HP, preventing a zero-HP user with a live substitute.
 local subUser = { mon = { stats = { hp = 40 }, hp = 10 }, name = "SUBBY" }
-MoveEffects.primary.SUBSTITUTE_EFFECT(sideRng, subUser)
-check(subUser.substituteHP ~= nil and subUser.mon.hp == 0,
-      "substitute built at exactly 1/4 max HP leaves 0 HP")
-local subUser2 = { mon = { stats = { hp = 40 }, hp = 9 }, name = "SUBBY" }
-local subMsgs = MoveEffects.primary.SUBSTITUTE_EFFECT(sideRng, subUser2)
-check(subUser2.substituteHP == nil
+local subMsgs = MoveEffects.primary.SUBSTITUTE_EFFECT(sideRng, subUser)
+check(subUser.substituteHP == nil and subUser.mon.hp == 10
       and subMsgs[1]:find("weak", 1, true) ~= nil,
+      "substitute fails at exactly 1/4 max HP")
+local subUser2 = { mon = { stats = { hp = 40 }, hp = 9 }, name = "SUBBY" }
+local subMsgs2 = MoveEffects.primary.SUBSTITUTE_EFFECT(sideRng, subUser2)
+check(subUser2.substituteHP == nil and subUser2.mon.hp == 9
+      and subMsgs2[1]:find("weak", 1, true) ~= nil,
       "substitute fails below 1/4 max HP")
 
 -- Haze clears Disable/X ACCURACY on both sides and forfeits the turn of