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/core/SaveData.lua

@@ -241,6 +241,11 @@ function SaveData.defaultOptions()
     videoMode = "windowed",
     -- hard render frame-rate cap; render-only pacing (issue #88, FrameCap.lua)
     fpsCap = 60,
+    -- graphics performance tier: auto | high | balanced | low.  "auto"
+    -- picks a default from the device (ARM handhelds/phones drop the heavy
+    -- extras); scales TILT / GBC FX / survey ZOOM / FPS but never game
+    -- logic.  See src/core/Performance.lua.
+    performance = "auto",
     -- Per-pipeline display levels, keyed by render_pipelines id (see
     -- src/render/Pipelines.lua).  A level for a mod that is not installed
     -- is kept rather than pruned, so re-enabling the mod restores the mode