Os.heap_relief(): the allocator's freed-but-cached memory handed back, once after a load

Beside Os.heap_bytes: malloc_zone_pressure_relief(NULL, 0) on macOS (weak, so a libc without it reads
0) and HeapCompact(GetProcessHeap(), 0) on Windows - kernel32 only, so the Windows build imports
nothing new; the bytes it says it released. Once after a load, never per frame.

Measured, for the record: on macOS it does NOT reach the large-block cache. A C program that frees six
15 MB blocks still holds 90 MB of MALLOC_LARGE (empty) after relief on every zone (it returns 0); only
MallocLargeCache=0 in the environment AT PROCESS START turns the cache off (read at malloc's init -
set later, it does nothing). Maroon Lake's watcher sets it for the processes it spawns.

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

selfhost/backend/emit_decl.ludic

@@ -361,6 +361,7 @@ function emit_program() -> void {
   if g_uses_osrt { emit_os_prelude() }           # @lp_os_args/platform/arch/save_dir/... (libc env + uname)
   if g_uses_pid and (not g_target_win) { emith("declare i32 @getpid()\n") }
   if g_uses_heap { emit_os_heap() }
+  if g_uses_relief { emit_os_relief() }
   if g_uses_unicodert { emit_unicode_prelude() }  # @lp_uni_len/valid/decode/case/truncate/grapheme (UTF-8)
   if g_uses_fsrt { emit_fs_prelude() }           # @fn_fs_*/fn_path_*/fn_mime_* (libc + string ops)
   if g_uses_pak { emit_pak_prelude() }           # @lp_pak_* asset packs + the pre-main mount ctor