build(justfile): cap the fuzz recipe's workers

Assisted-by: GLM 5.3 Flash

justfile

@@ -23,6 +23,12 @@ memlimit := "4G"
 # 2026-10-02), and the wide machine at the keyboard can carry it.
 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"
+
 bindir := env_var_or_default("BINDIR", env_var("HOME") / ".local" / "bin")
 
 # Where `install-man` puts the gzip-compressed pages (man1 below it). Exported because
@@ -69,7 +75,7 @@ unit pkgs=packages run=".*":
 
 # Time-boxed fuzz of one target in one package. The package is required; never a gate.
 fuzz target pkg fuzztime="60s":
-    systemd-run --user --scope -p MemoryMax={{memlimit}} -p MemorySwapMax=0 go test -timeout 0 -run '^$' -fuzz '{{target}}' -fuzztime={{fuzztime}} {{pkg}}
+    systemd-run --user --scope -p MemoryMax={{memlimit}} -p MemorySwapMax=0 go test -timeout 0 -run '^$' -fuzz '{{target}}' -fuzztime={{fuzztime}} -parallel={{fuzzparallel}} {{pkg}}
 
 # The cmd/link GOOBJ parity gate, opt-in: it costs minutes no pipeline can afford.
 link-parity: