perf(retrieval): optimize map-nav evidence packing

apps/api/tests/contract/test_retrieval_lazy_tree_contract.py

@@ -5,6 +5,9 @@
 from shared.services.retrieval.nav.nav_hierarchy import NodeMeta, ProviderToolSpace
 from shared.services.retrieval.nav.nav_knowhere import KnowhereProvider, SectionRow
 from shared.services.retrieval.nav.nav_map_scores import _walk_tree
+from shared.services.retrieval.nav._compat import Chunk
+from shared.services.retrieval.nav.nav_compose import pack_nav_evidence
+from shared.services.retrieval.nav.nav_types import NavConfig, NavState
 
 
 class _MetadataForbiddenProvider(KnowhereProvider):
@@ -31,3 +34,72 @@ def test_tree_walk_reads_children_and_titles_without_materializing_metadata() ->
     assert children == {"root": ["child"], "child": []}
     assert leaves == {"child"}
     assert titles == {"root": "Root", "child": "Child"}
+
+
+def test_evidence_pack_reads_titles_without_materializing_subtree_metadata() -> None:
+    class _CountingMetadataProvider(_MetadataForbiddenProvider):
+        metadata_calls = 0
+
+        def node_meta(self, section_id: str) -> NodeMeta:
+            self.metadata_calls += 1
+            return super().node_meta(section_id)
+
+    provider = _CountingMetadataProvider(
+        doc_id="doc",
+        sections=[
+            SectionRow("root", None, "Root", "Root", 0, "", 0),
+            SectionRow("child", "root", "Root / Child", "Child", 1, "", 1),
+        ],
+        units=(),
+    )
+    toolspace = ProviderToolSpace(provider)
+    state = NavState(doc_id="doc", query="child")
+    chunk = Chunk(
+        node_id="child",
+        doc_id="doc",
+        text="evidence",
+        line_ids=(1,),
+        section_id="child",
+    )
+
+    result = pack_nav_evidence(
+        [(chunk, 1.0)],
+        toolspace,
+        state,
+        NavConfig(),
+        budget_chars=100,
+    )
+
+    assert result.evidence_text == "[E1]\n[§ Child]\nevidence"
+    assert provider.metadata_calls == 0
+
+
+def test_evidence_pack_identifies_header_owners_from_parent_chain() -> None:
+    provider = KnowhereProvider(
+        doc_id="doc",
+        sections=[
+            SectionRow("root", None, "Root", "Root", 0, "", 0),
+            SectionRow("parent", "root", "Root / Parent", "Parent", 1, "", 1),
+            SectionRow(
+                "child", "parent", "Root / Parent / Child", "Child", 2, "", 2
+            ),
+        ],
+        units=(),
+    )
+    toolspace = ProviderToolSpace(provider)
+    state = NavState(doc_id="doc", query="child")
+    chunks = [
+        Chunk("parent", "doc", "parent evidence", (1,), "parent"),
+        Chunk("child", "doc", "child evidence", (2,), "child"),
+    ]
+
+    result = pack_nav_evidence(
+        [(chunks[0], 1.0), (chunks[1], 0.9)],
+        toolspace,
+        state,
+        NavConfig(),
+        budget_chars=200,
+    )
+
+    assert result.kept_chunks == [chunks[1]]
+    assert result.evidence_text == "[E1]\n[§ Child]\nchild evidence"