crawl central-db + query auto-include + read-seam provenance

- make crawl-ingest writes to one central crawl db (CRAWL_DB, default
  ~/.arborist/crawl/web.db) instead of per-domain shards in the
  peer-shared main dir: keeps locally-crawled content out of peer
  sharing by default and a growing domain set under SQLite's 10-attach
  cap (Makefile, docs/crawler.md).

- arborist query auto-includes the local crawl db (query() gains
  extra_shards; CLI --include-shard / --no-crawl-db, default-on when
  web.db exists). Fix latent --db single-file query AttributeError
  (cli.py). Persist used / used_pointer_ids + retrieval_purity into
  merkle_proof so read-only consumers can see which chunks fed the
  answer (qa/query.py).

- arborist.read: read-only seam for dashboards / verifiers; on a
  multi-source context root surface the real primary source instead of
  the opaque corpus://multi-source sentinel (read.py). Backs the
  arborist-viz Merkle Command Center (#000069).

- tests for extra_shards, the CLI crawl-db resolver, and the read seam.

tests/test_cli_render.py

@@ -383,3 +383,40 @@ def test_render_label_format_collapsed_surfaces_tail():
     out = _render_query_human(r, "winners of all major sports?")
     assert "format collapsed" in out
     assert "UNGROUNDED" in out
+
+
+# --- auto-include of the local crawl db -----------------------------
+
+
+def test_resolve_extra_shards_auto_includes_crawl_db(tmp_path, monkeypatch):
+    """`arborist query` auto-includes the local crawl db (when present),
+    honors --no-crawl-db, and appends --include-shard extras de-duped."""
+    import argparse
+
+    from arborist.cli import _default_crawl_db, _resolve_extra_shards
+
+    crawl_db = tmp_path / "web.db"
+    crawl_db.write_bytes(b"")  # resolver only checks presence
+    monkeypatch.setenv("ARBORIST_CRAWL_DB", str(crawl_db))
+    assert _default_crawl_db() == crawl_db
+
+    # present → auto-included
+    ns = argparse.Namespace(no_crawl_db=False, include_shard=None)
+    assert _resolve_extra_shards(ns) == [crawl_db]
+
+    # --no-crawl-db opts out
+    ns_off = argparse.Namespace(no_crawl_db=True, include_shard=None)
+    assert _resolve_extra_shards(ns_off) == []
+
+    # explicit --include-shard appends extras; crawl db deduped
+    extra = tmp_path / "extra.db"
+    extra.write_bytes(b"")
+    ns_inc = argparse.Namespace(
+        no_crawl_db=False, include_shard=[str(extra), str(crawl_db)]
+    )
+    assert _resolve_extra_shards(ns_inc) == [crawl_db, extra]
+
+    # missing crawl db → silently dropped
+    monkeypatch.setenv("ARBORIST_CRAWL_DB", str(tmp_path / "nope.db"))
+    ns_missing = argparse.Namespace(no_crawl_db=False, include_shard=None)
+    assert _resolve_extra_shards(ns_missing) == []