feat: LibriVox/NASA/Internet Archive fetchers (+ deferred stubs)

Per sub-project-2 plan Task 9 / spec §6.4. Three keyless first-ship fetchers
parse documented JSON APIs via an injected HttpClient; license/attribution go
through tools.licensing; NASA third-party + IA no-license cases are flagged in
notes for review. musopen/fma/freesound are explicit deferred stubs raising
NotImplementedError; freesound documents FREESOUND_API_TOKEN (secret, env-only).

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

tools/ingest/musopen.py

@@ -0,0 +1,25 @@
+"""Musopen fetcher — DEFERRED (see spec §6.4).
+
+Public-domain / CC classical music. Access has historically been gated and may
+require an API key (a secret — never logged, see spec §9). The Fetcher seam is
+wired so enabling it later is purely additive; it raises until implemented.
+"""
+
+from __future__ import annotations
+
+from tools.ingest.base import Candidate
+
+_DEFERRED = "deferred — see spec §6.4 (Musopen access may require an API key)"
+
+
+class MusopenFetcher:
+    archive = "musopen"
+
+    def __init__(self, client):
+        self.client = client
+
+    def search(self, query: str, *, limit: int) -> list[Candidate]:
+        raise NotImplementedError(_DEFERRED)
+
+    def resolve(self, identifier: str) -> Candidate:
+        raise NotImplementedError(_DEFERRED)