fix: write one truth model per target, not alias

generate_truth named a narrowed model's directory after the target
as typed, so three aliases of one target wrote three copies, and
the pipeline rebuilt the slug by hand to diff it. One function now
names the directory from the canonical target for both.

scripts/generate_truth.py

@@ -10,7 +10,6 @@ from __future__ import annotations
 
 import argparse
 import os
-import re
 import sys
 
 sys.path.insert(0, os.path.dirname(__file__))
@@ -21,6 +20,7 @@ from common import (
     load_platform_config,
     load_target_config,
     require_yaml,
+    truth_target_dir,
     write_text_atomic,
     yaml_load,
 )
@@ -94,8 +94,9 @@ def main(argv: list[str] | None = None) -> None:
     # compared a narrowed model against the whole scrape.
     output_dir = args.output_dir
     if args.target:
-        slug = re.sub(r"[^a-z0-9]+", "-", args.target.strip().lower()).strip("-")
-        output_dir = os.path.join(output_dir, slug or "target")
+        output_dir = truth_target_dir(
+            output_dir, platforms, args.target, args.platforms_dir
+        )
     os.makedirs(output_dir, exist_ok=True)
 
     for name in platforms: