fix(core): cut per-frame allocation churn and GC pressure on the main loop

On weak single-core handhelds (RK3326/RG351MP) the main thread sits at
100% CPU, so every millisecond of jitter drops a frame: constant
stuttering while walking even at PERFORMANCE LOW with all mods off,
while average CPU usage stays unchanged.  Three sources of per-frame
garbage and pause on the hot path:

- Game:step / Game:logicSpeed passed inline closures to ModRuntime.call,
  allocating a fresh function 60 times per second.  Hoist both to
  module-level locals; behavior is identical.
- Game:update advanced the incremental collector every rendered frame.
  Space it to every 4th frame: the explicit frees the comment refers
  to still do the heavy lifting, collection stays incremental, and the
  stepping itself stops competing with the 16.6ms frame budget.
- checkEmergencyQuit called love.joystick.getJoysticks() (a fresh
  table) twice per frame to guard a 5-second hold combo.  Cache the
  joystick list and refresh it once per second; a 1s hotplug delay is
  irrelevant against a 5s hold.

Tested on an RG351MP (RK3326, dArkOS, v0.2.41): eliminates the
constant walking stutter with all mods off, and with 11 mods
re-enabled.  Average CPU is unchanged (the cost was variance, not
load), which matches the diagnosis.

main.lua

@@ -34,12 +34,21 @@ end
 
 -- Global emergency quit: holding Start + Select for 5 seconds forcefully terminates LOVE.
 local emergencyQuitTimer = 0
+-- getJoysticks() allocates a fresh table every call; this runs once (twice)
+-- per frame, so cache the list and refresh it once a second instead.  The 5s
+-- hold requirement makes a 1s hotplug delay irrelevant.
+local cachedJoysticks = nil
+local joystickCacheAge = 1
 
 local function checkEmergencyQuit(dt)
   local held = false
+  joystickCacheAge = joystickCacheAge + (dt or 0.016)
   if love.joystick and love.joystick.getJoysticks then
-    local joysticks = love.joystick.getJoysticks()
-    for _, j in ipairs(joysticks) do
+    if not cachedJoysticks or joystickCacheAge >= 1 then
+      cachedJoysticks = love.joystick.getJoysticks()
+      joystickCacheAge = 0
+    end
+    for _, j in ipairs(cachedJoysticks) do
       if j:isGamepad() then
         local start = j:isGamepadDown("start")
         local selectBtn = j:isGamepadDown("back") or j:isGamepadDown("guide")

src/core/Game.lua

@@ -21,6 +21,12 @@ local function renderVisible(stack, state)
   return state and (not stack.renderVisible or stack:renderVisible(state))
 end
 
+-- Vanilla defaults for ModRuntime.call, hoisted to module level: called from
+-- the 60Hz logic step / per-frame speed resolution, an inline closure here
+-- allocated a fresh function every tick for no behavioral gain.
+local function noop() end
+local function resolveLogicSpeedVanilla(g) return g:_resolveLogicSpeed() end
+
 -- dev-mode gate for the F5/backtick hotkeys; false keeps every src/dev
 -- module unloaded, so a player boot never touches a byte of dev code
 local devMode = os.getenv("POKEPORT_DEV") == "1" or _G.POKEPORT_DEV_MODE == true
@@ -304,7 +310,7 @@ function Game:step(dt)
   -- the same fixed-step boundary as a physical controller.  Run them before
   -- Input:step promotes queued edges so a button chosen here is visible to
   -- this logic tick, not one tick later.  With no wrapper this is a no-op.
-  ModRuntime.call("input.step", function() end, self, dt)
+  ModRuntime.call("input.step", noop, self, dt)
   self.input:step()
   -- A+B+SELECT+START held for 16 steps: SoftReset (home/init.asm) stops the
   -- audio, whites the palettes out and falls through into Init, i.e. the
@@ -390,7 +396,7 @@ function Game:logicSpeed()
   -- returns a bad value, so an unclamped result would flow straight into
   -- the FixedStep accumulator math below and freeze or destabilize logic.
   return GameSpeed.clamp(ModRuntime.call("core.logic_speed",
-    function(g) return g:_resolveLogicSpeed() end, self))
+    resolveLogicSpeedVanilla, self))
 end
 
 function Game:update(dt)
@@ -420,12 +426,15 @@ function Game:update(dt)
   pcall(function() require("src.core.DiscordPresence").update(dt) end)
   self:updateSync(dt)
   -- Steady-state memory backstop: advance the incremental collector one
-  -- small step every rendered frame.  The heavy GPU objects are now freed
-  -- explicitly (map eviction, battle exit, canvas/renderer swaps), so this
-  -- only has to keep ordinary Lua-heap garbage (per-frame tables/closures)
-  -- from drifting upward over a long session, and to spread collection out
-  -- so the default lazy schedule never batches it into a visible pause.
-  if collectgarbage then collectgarbage("step", 1) end
+  -- small step every few rendered frames.  The heavy GPU objects are now
+  -- freed explicitly (map eviction, battle exit, canvas/renderer swaps), so
+  -- this only has to keep ordinary Lua-heap garbage (per-frame tables) from
+  -- drifting upward over a long session, and to spread collection out so the
+  -- default lazy schedule never batches it into a visible pause.  Every 4th
+  -- frame (not every frame) so the stepping itself does not compete with
+  -- the frame budget on weak single-core handhelds.
+  self.gcStepFrame = (self.gcStepFrame or 0) + 1
+  if collectgarbage and self.gcStepFrame % 4 == 0 then collectgarbage("step", 1) end
 end
 
 -- render.zones' identity default: unhooked, the zone list reaches the blit