feat: per-archive license normalization

Per sub-project-2 plan Task 6 / spec §5.3. normalize_license maps CC URLs /
identifiers and public-domain markers to the LICENSES vocab, builds non-empty
attribution for cc_by/cc_by_nc, and rejects unmappable licenses at ingest.
librivox_license() helper returns public_domain.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/licensing.py

@@ -0,0 +1,63 @@
+"""Normalize per-archive origin license metadata to the hef.catalog LICENSES vocab.
+
+Maps Creative Commons URLs / identifiers and public-domain markers to
+{public_domain, cc0, cc_by, cc_by_nc}. For the attribution licenses (cc_by,
+cc_by_nc) it builds a non-empty attribution string — hef.catalog.validate()
+rejects those licenses with an empty attribution. Anything unmappable is
+rejected (a piece whose license can't be established is not ingested).
+"""
+
+from __future__ import annotations
+
+import re
+
+
+def _attribution(creator: str, license_name: str, raw: str, default_label: str) -> str:
+    label = license_name or default_label or raw
+    parts = []
+    if creator:
+        parts.append(creator)
+    parts.append(label)
+    return " — ".join(parts)
+
+
+def normalize_license(raw, *, creator="", license_name=""):
+    """Return (license, attribution) for an origin license string.
+
+    Raises ValueError when `raw` does not map to an allowed license.
+    """
+    text = (raw or "").strip().lower()
+    if not text:
+        raise ValueError(f"unmappable license: {raw!r}")
+
+    # CC0 / public-domain dedication (check before the generic publicdomain
+    # markers, since the CC0 URL also contains "publicdomain").
+    if "publicdomain/zero" in text or re.search(r"\bcc[ _-]?0\b", text):
+        return ("cc0", "")
+
+    # Public Domain Mark / explicit public domain / no-known-copyright.
+    pd_markers = (
+        "publicdomain/mark",
+        "public_domain",
+        "public domain",
+        "publicdomain",
+        "no known copyright",
+        "pdm",
+    )
+    if any(m in text for m in pd_markers):
+        return ("public_domain", "")
+
+    # CC BY-NC must be checked before the bare CC BY.
+    if re.search(r"by[ _-]?nc", text):
+        return ("cc_by_nc", _attribution(creator, license_name, raw, "CC BY-NC"))
+
+    # CC BY.
+    if re.search(r"\bcc[ _-]?by\b", text) or "licenses/by" in text:
+        return ("cc_by", _attribution(creator, license_name, raw, "CC BY"))
+
+    raise ValueError(f"unmappable license: {raw!r}")
+
+
+def librivox_license():
+    """LibriVox recordings are public domain by charter (attribution not required)."""
+    return ("public_domain", "")