Cap the auto-sized JVM heap against total device RAM

[jfrolund]

Problem

The auto path sizes the JVM heap purely from free memory:

long availMem = (ami.availMem-ami.threshold)/(1024*1024);
long allocatedRam = Math.max(availMem, 1536);

That is a floor with no ceiling, so the more RAM is free at launch, the larger a heap
the game claims for the rest of the session. 874e85b ("Fix ram setter") introduced
this when it replaced the previous bounded calculation, which did cap the result. On
this branch the *= 0.8 discount is gone as well, so nothing tempers the reading at all.

The relationship is backwards on a headset. The heap is only part of the process — the
GL driver, XR swapchains, LWJGL and the JRE need roughly as much again outside it — and
the VR compositor and system services have to keep running or lmkd kills the game.

Evidence

Measured on a 6GB Quest 2 (Horizon OS, MC 1.21.5 + Vivecraft):

Free RAM at launch	Heap granted	Process PSS	Result
~322 MB	1536 MB	3.1 GB	survived
~3.5 GB	2048 MB	4.24 GB	repeated reason=3 (LOW_MEMORY) kills

Having more memory free made the game less stable, not more. With the heap held to
1456 MB instead, the same device measured 3.12 GB PSS and 754 MB free, with no
LOW_MEMORY kills across ~21 hours and several sessions.

On this branch the same 3.5 GB-free launch would request roughly a 3300 MB heap.

Change

• Clamp -Xmx to a quarter of total RAM. This scales with the device rather than
  hardcoding a number, and makes the heap stable across launches instead of varying
  with whatever else happened to be running.
• Lower the startup commitment so the heap grows only as the game needs it, rather than
  committing 1024 MB in every session.
• Stop pinning -Xms to -Xmx on the custom path, where it forced the user's entire
  chosen value to be committed immediately.

Resulting caps: Quest 2 → 1458 MB, Quest 3 → ~1950 MB, 12GB Quest Pro → ~2925 MB.

The GC flags are deliberately untouched. -XX:+UseZGC carries real native overhead and
is a tempting target, but it was presumably chosen for frame-time consistency, which
matters more here than in a desktop game — that is a separate argument needing separate
evidence.

Testing caveat

Compiles against QuestCraft-6.0.1 (JDK 17). The on-device numbers above come from
patching the heap constants in the shipped QuestCraft 6.0.0 APK, whose Pojlib predates
874e85b — so they validate the effect of bounding the heap rather than this exact
code path. I do not have a Unity build environment to produce a full APK from this
branch; happy to adjust if you can run it through the normal pipeline.

🤖 Generated with Claude Code