feat: version-lineage report in the crawler ingestion pipeline

When a re-crawl detects a real content delta (a just-ingested root that
supersedes a prior version — content hash changed, not redeploy/ETag
noise the idempotent ingest already no-op'd), the pipeline now surfaces
the page's document chain over time instead of just 'something changed'.

bridge.py: version_chain(conn, uri) walks a URI's documents by ingest_ts
(each content change = new content-addressed doc + supersedes edge);
delta_report() adds the word-level similarity of the latest change;
render_delta_report() prints it. ingest_crawled() detects superseding
roots, emits the lineage report to stderr per changed page, and returns
'deltas' in its summary. Validated on the live russell.ballestrini.net
re-crawl: 223 pages, full redeploy, exactly 1 content change (/about/),
rendered as a 2-version chain (90% similar to prior). 2 tests; suite
2551 passed.

tests/test_version_lineage.py

@@ -0,0 +1,73 @@
+"""Version lineage in the crawler ingestion pipeline (2026-05-21).
+
+A content change to a URI writes a new content-addressed document + a
+``supersedes`` edge, so a URI accumulates a chain of versions over time.
+A redeploy that doesn't change content is a no-op (same Merkle root) and
+adds NO version. The crawler emits a lineage report when it detects a
+real delta. See arborist/sources/crawler/bridge.py.
+"""
+from __future__ import annotations
+
+from typing import Iterator
+
+from arborist.document import Document
+from arborist.ingest import ingest_source
+from arborist.source import Source
+from arborist.sources.crawler.bridge import (
+    delta_report,
+    render_delta_report,
+    version_chain,
+)
+from arborist.store import connect
+
+
+class FakeSource(Source):
+    source_type = "fake"
+
+    def __init__(self, docs):
+        self.docs = docs
+
+    def iter_documents(self) -> Iterator[Document]:
+        yield from self.docs
+
+
+def _doc(uri: str, content: str) -> Document:
+    return Document(uri=uri, content=content, source_type="fake",
+                    title="page", edges=[])
+
+
+def test_content_change_builds_a_version_chain(tmp_path):
+    conn = connect(tmp_path / "t.db")
+    uri = "test://page"
+    ingest_source(conn, FakeSource([_doc(uri, "alpha bravo charlie delta echo")]))
+    # changed content, same URI -> new root + supersedes edge
+    ingest_source(conn, FakeSource(
+        [_doc(uri, "alpha bravo charlie delta ECHO foxtrot golf")]))
+
+    chain = version_chain(conn, uri)
+    assert len(chain) == 2  # two versions over time
+
+    rep = delta_report(conn, uri)
+    assert rep["n_versions"] == 2
+    assert "latest_delta" in rep
+    assert 0.0 < rep["latest_delta"]["similarity"] < 1.0  # changed, not total
+
+    txt = render_delta_report(rep)
+    assert "content changed" in txt
+    assert "v1" in txt and "v2" in txt
+    assert "similar to prior" in txt
+
+
+def test_identical_recrawl_adds_no_version(tmp_path):
+    # A redeploy with byte-identical content must NOT create a new version
+    # (content-addressed idempotence — the whole point).
+    conn = connect(tmp_path / "t.db")
+    uri = "test://stable"
+    ingest_source(conn, FakeSource([_doc(uri, "same content stays the same")]))
+    ingest_source(conn, FakeSource([_doc(uri, "same content stays the same")]))
+
+    chain = version_chain(conn, uri)
+    assert len(chain) == 1  # no-op re-ingest -> still one version
+    rep = delta_report(conn, uri)
+    assert rep["n_versions"] == 1
+    assert "latest_delta" not in rep