Os.platform/arch uname once; render3d primes a new buffer's Metal buffer

lp_os_platform and lp_os_arch malloc'd 8 KB per call (the uname buffer) and kept none of it:
string_temps now asks both every round, 327 MB over 20,000 before, 0 after. Reseeded. render3d:
MoltenVK made a mapped buffer's MTLBuffer at its first bind (fn_gvk_draw +4 blocks in the boat
window); gvk_buf_reserve queues it and the next frame's command buffer copies 4 bytes out of it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

changes/moltenvk-no-command-pooling.md

@@ -6,3 +6,5 @@ busiest frame so far, for as long as the game ran. render3d turns it off
 (`MVK_CONFIG_USE_COMMAND_POOLING=0`, unless the environment already says otherwise) before the first
 Vulkan call; the objects are made and freed with their command buffer, at no measured cost.
 `examples/rendering/steady.ludic` ramps a frame from 20 to 200 actors and fails on what pooling left.
+A buffer written through its mapping is also read once at the start of the next frame's commands,
+so MoltenVK makes its Metal buffer then rather than at its first draw, however much later that is.

changes/os-platform-once.md

@@ -0,0 +1,5 @@
+bump: patch
+type: fix
+**`Os.platform()` and `Os.arch()` allocate nothing after the first call.** Each call malloc'd an
+8 KB `uname` buffer and let it go, and a game asks the platform every frame in places (a launcher's
+wait, an update panel, a renderer's present). The buffer is made once and kept.