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/common.py

@@ -487,6 +487,21 @@ def canonical_target_name(
     return found.pop() if len(found) == 1 else target
 
 
+def truth_target_dir(
+    output_dir: str, platforms: list[str], target: str, platforms_dir: str = "platforms"
+) -> str:
+    """Where a target-narrowed truth model is written and read.
+
+    Slugged from the name the target is filed under: `nx`, `switch` and
+    `nintendo-switch` wrote three copies of one model, and a run under one
+    alias left the others stale beside it. generate_truth writes here and
+    the pipeline's diff reads here, so both call this.
+    """
+    named = canonical_target_name(platforms, target, platforms_dir)
+    slug = re.sub(r"[^a-z0-9]+", "-", named.strip().lower()).strip("-")
+    return os.path.join(output_dir, slug or "target")
+
+
 def load_target_config(
     platform_name: str,
     target: str,