support proper 32-byte RNG seeds

• add optional $.help to argument err of error throw
• parse prng_seed: [u8; 32] in rng and inbreeding
• compute the "local" seed by hashing the "global" seed, the strategy and the iteration index
• pass --prng-seed: string, a 64-char long seed to inbreeding run

bins/inbreeding/run.nu

 use consts.nu
 use ../../.nushell cargo "cargo bin"
+use ../../.nushell error "error throw"
+
+const VALID_HEX_CHARS = "abcdefABCDEF0123456789"
+
+def check-hex [-n: int]: [
+    string -> record<
+        ok: bool,
+        err: record<msg: string, label: string, help: string>,
+    >
+] {
+    let s = $in
+
+    if ($s | str length) != $n {
+        return {
+            ok: false,
+            err: {
+                msg: "invalid HEX length"
+                label : $"length is ($s | str length)",
+                help: "length should be 64",
+            },
+        }
+    }
+
+    for c in ($s | split chars | enumerate) {
+        if not ($VALID_HEX_CHARS | str contains $c.item) {
+            return {
+                ok: false,
+                err: {
+                    msg: "bad HEX character",
+                    label: $"found '($c.item)' at ($c.index)",
+                    help: $"expected one of '($VALID_HEX_CHARS)'",
+                },
+            }
+        }
+    }
+
+    { ok: true, err: {} }
+}
 
 export def main [
     --options: record<
@@ -14,7 +52,7 @@ export def main [
         strategies: list<string>,
         environment: string,
     >,
-    --prng-seed: int = 0,
+    --prng-seed: string = "0000000000000000000000000000000000000000000000000000000000000000",
 ] {
     if $options.measurement_schedule_start > $options.max_t {
         error make --unspanned {
@@ -22,6 +60,16 @@ export def main [
         }
     }
 
+    let res = $prng_seed | check-hex -n 64
+    if not $res.ok {
+        error throw {
+            err: $res.err.msg,
+            label: $res.err.label,
+            span: (metadata $prng_seed).span,
+            help: $res.err.help,
+        }
+    }
+
     let now = date now | format date "%s%f"
 
     for s in $options.strategies {