bench-spec-ab: sweep an impl's own window, not just the spec type

ngram-mod's match window (n_min/n_max) decides which traffic it can see at all
-- the fleet's 48/64 preset is tuned for long verbatim echoes and finds nothing
in a repetitive-but-short output. Config entries now take |KEY=VALUE overrides,
so the window can be swept in the same run as the configurations it applies to.

scripts/bench-spec-ab.py

@@ -96,41 +96,35 @@ def log(msg, logfile):
 
 def sh(cmd):
     return subprocess.run(cmd, shell=True, capture_output=True, text=True).stdout.strip()
-
-
 def parse_configs(spec):
+    """`name=SPEC_TYPE[@n_max][|KEY=VALUE]...`, entries joined by ';'.
+
+    KEY=VALUE parts override any env file key, which is how an impl's own
+    window (SPEC_NGRAM_N_MIN/MAX, for instance) gets swept without hand-editing
+    env files."""
     out = []
     for chunk in spec.split(";"):
         chunk = chunk.strip()
         if not chunk:
             continue
         name, _, rest = chunk.partition("=")
+        over = {}
+        if "|" in rest:
+            rest, _, extras = rest.partition("|")
+            for item in extras.split("|"):
+                if "=" in item:
+                    k, _, v = item.partition("=")
+                    over[k.strip()] = v.strip()
         n_max = None
         if "@" in rest:
             rest, _, n_max = rest.partition("@")
-        over = {"SPEC_TYPE": rest.strip()}
+        over["SPEC_TYPE"] = rest.strip()
         if n_max:
             over["SPEC_DRAFT_N_MAX"] = n_max.strip()
         out.append((name.strip(), over))
     return out
 
 
-def write_env(src, name, overrides):
-    path = "/tmp/spec-ab-" + name.replace("/", "_") + ".env"
-    with open(src) as fh:
-        lines = fh.read().splitlines()
-    out = []
-    for line in lines:
-        for k, v in overrides.items():
-            if line.startswith(k + "="):
-                line = k + "=" + v
-                break
-        out.append(line)
-    with open(path, "w") as fh:
-        fh.write("\n".join(out) + "\n")
-    return path
-
-
 def post(base, body, timeout=1800):
     req = urllib.request.Request(
         base + "/v1/chat/completions", data=json.dumps(body).encode(),