fix: description placeholder sentinel leaking to output

Two bugs:
1. Sentinel used double-underscore delimiters (__DESC:id__) — fragile regex
   [^_]+ would break on IDs containing underscores, and Pass-1 output
   containing sentinels would leak through Pass 2 unstripped.
2. After Pass 2 substitution, sentinels inside substituted text (from
   Pass-1 output of the referenced id) were never cleaned up.

Fix: switch to null-byte delimiters (\x00DESC:id\x00) which cannot appear
in user text, and add a second .replace(SENTINEL_RE, '') pass to strip any
sentinels that survive after substitution (self-refs, nested cycles).

data/units.json

@@ -6,7 +6,8 @@
       "labelPlural": "pounds",
       "symbol": "lb",
       "group": "dickloads",
-      "toBase": 0.038
+      "toBase": 0.038,
+      "description": "{dickload:description}"
     },
     {
       "id": "dickload",
@@ -14,7 +15,8 @@
       "labelPlural": "dickloads",
       "symbol": "dl",
       "group": "dickloads",
-      "toBase": 1
+      "toBase": 1,
+      "description": "See {pound:description}"
     },
     {
       "id": "barrel",
@@ -69,24 +71,24 @@
     },
     {
       "id": "assload",
-      "label": "Assload",
-      "labelPlural": "Assloads",
+      "label": "assload",
+      "labelPlural": "assloads",
       "symbol": "assl",
       "group": "dickloads",
       "toBase": 2
     },
     {
       "id": "shitload",
-      "label": "Shitload",
-      "labelPlural": "Shitloads",
+      "label": "shitload",
+      "labelPlural": "shitloads",
       "symbol": "shtld",
       "group": "dickloads",
       "toBase": 12.6666666667
     },
     {
       "id": "shit-ton",
-      "label": "Shit ton",
-      "labelPlural": "Shit tons",
+      "label": "shit ton",
+      "labelPlural": "shit tons",
       "symbol": "shttn",
       "group": "dickloads",
       "toBase": 38