Add graphics performance tier for low-end devices

Introduce an OPTIONS -> PERFORMANCE setting that scales the port's
optional presentation extras down for weaker hardware, so older/lower-end
devices can run the game smoothly.

The tier governs the three heaviest non-faithful extras -- the 3D TILT,
the GBC FX post-process shader, and survey ZOOM (which renders connected
neighbor maps) -- plus a hard FPS ceiling. It never touches game logic,
which is fixed-step off dt, so every tier plays identically.

- src/core/Performance.lua: tiers (auto/high/balanced/low), a conservative
  device auto-detect (ARM handhelds -> low, phones -> balanced, normal
  desktops -> high), per-tier caps, and the option-row cycle. Zero
  requires, like GameVersion.
- Game:applyOptions clamps the *live* presentation state against the tier
  without rewriting stored options, so a lower tier hides the player's
  TILT/GBC FX/ZOOM/FPS choices and a higher tier restores them exactly.
- Zoom.offsetRange floors the range at FIT when survey is disallowed, so
  the option row, hotkey, and mouse wheel all stop at close-up on LOW.
- New save.options.performance default "auto"; OPTIONS row heads the
  display group and re-applies live.
- Tests: tests/engine/performance_tiers.lua (ROM-free); mod_ui_tests row
  golden updated for the spliced row.

AUTO resolves to HIGH on a normal desktop and on every options.lua that
predates the option, so the common case is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q6bFAiQyZ5jDmewsbB4LG9

src/ui/OptionsMenu.lua

@@ -20,6 +20,7 @@ local GameSpeed = require("src.core.GameSpeed")
 local GameVersion = require("src.core.GameVersion")
 local VideoMode = require("src.core.VideoMode")
 local FrameCap = require("src.core.FrameCap")
+local Performance = require("src.core.Performance")
 local Logger = require("src.core.Logger")
 local Runtime = require("src.mods.Runtime")
 local OptionRows = require("src.ui.OptionRows")
@@ -202,6 +203,21 @@ local function buildRows(game)
         require("src.core.Music").setFilterLevel(o.musicFilter)
         return true
       end },
+    -- Heads the port's display group: one tier that scales the heavy extras
+    -- (TILT / GBC FX / survey ZOOM) and the FPS ceiling for weaker devices.
+    -- AUTO picks a default from the hardware; every tier is overridable.
+    -- Re-applies live so the extras clamp (or, on a higher tier, restore to
+    -- the player's stored TILT / GBC FX / ZOOM) the moment the row changes.
+    { id = "performance", label = Strings("PERFORMANCE"),
+      value = function(g)
+        return Strings(Performance.label(g.save.options.performance))
+      end,
+      step = function(g, dir)
+        local o = g.save.options
+        o.performance = Performance.cycle(o.performance, dir)
+        g:applyOptions(o)
+        return true
+      end },
     { id = "colors", label = Strings("COLORS"),
       value = function(g)
         return PaletteFX.modeLabel(g.save.options.colors or "gbc")