build(justfile): lower the fuzz worker default to the fence the assembler target holds

Assisted-by: GLM 5.3 Flash

justfile

@@ -26,8 +26,11 @@ racememlimit := "16G"
 # The fuzz recipe's worker cap. The default -parallel over this machine's cores
 # bursts through the 4G fence within seconds (measured: 32 workers take the OOM
 # kill, 12 peak near 2G and ran a 20-minute campaign without touching the
-# ceiling), so the workers ride a variable a longer campaign can override.
-fuzzparallel := "12"
+# ceiling), so the workers ride a variable a longer campaign can override. The
+# assembler target is the exception the default cannot carry: twelve workers
+# overran the fence three times where four ran a 30-minute campaign clean
+# (peaking near 1G), so 4 is the safe value for every target.
+fuzzparallel := "4"
 
 bindir := env_var_or_default("BINDIR", env_var("HOME") / ".local" / "bin")