fix: publish target aliases and refuse an unknown one

The pack builder accepts the aliases declared in the target overrides,
so --target switch works there, but the installer's target manifests
carried only canonical names. The documented word was the one that
failed, and the installer then carried on with every file: 1911 files
and 4.1 GB where the user had asked for the 863 that target needs.

Aliases are emitted beside their canonical target, and an unknown
target now stops the run and lists what is available. A filter is
applied or refused, never ignored.

scripts/generate_pack.py

@@ -2798,6 +2798,23 @@ def generate_target_manifests(targets_dir: str, output_dir: str) -> None:
         targets = data.get("targets", {})
         if not isinstance(targets, dict):
             raise ValueError(f"{yml_file}: targets must be a mapping")
+
+        # The pack builder accepts the aliases declared in _overrides.yml, so
+        # --target switch works there. Emitting only canonical names left the
+        # installer rejecting the same word and, worse, carrying on with every
+        # file instead: the user asked for a filter and got the full pack.
+        overrides_path = targets_path / "_overrides.yml"
+        alias_map: dict[str, list[str]] = {}
+        if overrides_path.is_file():
+            with open(overrides_path) as f:
+                all_overrides = yaml_load(f) or {}
+            for tname, ovr in (
+                all_overrides.get(yml_file.stem, {}).get("targets", {}) or {}
+            ).items():
+                names = [a for a in (ovr or {}).get("aliases", []) if isinstance(a, str)]
+                if names:
+                    alias_map[tname] = names
+
         result: dict[str, list[str] | None] = {}
         for target_name, target_info in targets.items():
             if not isinstance(target_name, str) or not target_name:
@@ -2818,6 +2835,17 @@ def generate_target_manifests(targets_dir: str, output_dir: str) -> None:
                     "non-empty strings"
                 )
             result[target_name] = cores
+
+        for target_name, names in alias_map.items():
+            if target_name not in result:
+                continue
+            for alias in names:
+                if alias in result:
+                    raise ValueError(
+                        f"{yml_file}: alias {alias!r} collides with a target name"
+                    )
+                result[alias] = result[target_name]
+
         out_path = Path(output_dir) / f"{yml_file.stem}.json"
         with open(out_path, "w") as f:
             json.dump(result, f, indent=2, sort_keys=True)