api: emit canonical heading path (title_path) on treewalk citations

Citations carried only {start_page, end_page, section_ids, quote} — no
structural heading path. The bench's path_correct@1 reads a title_path it
could only reconstruct from the parser's chunk titles, which are the wrong
vocabulary, so the metric was structurally 0%.

buildTreeWalkCitations now resolves the document's LLM TOC tree (via the
strategy's TOC provider) into a section-ID -> heading-path map
(tree.BuildHeadingPaths, HAL-109) and attaches the primary section's
heading path as title_path on each citation. Degrades cleanly: no TOC
persisted -> field omitted, prior behaviour unchanged.

Part of HAL-70.

Author: hallelx2

internal/api/treewalk.go

@@ -310,6 +310,15 @@ func (d Deps) buildTreeWalkCitations(ctx context.Context, t *tree.Tree, res *ret
 	sources := retrieval.CitationSources(res)
 	citations := make([]map[string]any, 0, len(sources))
 
+	// Canonical heading paths (section ID → logical heading path), resolved
+	// from the document's LLM TOC tree by page-range containment (HAL-109).
+	// This is what makes a citation carry a real STRUCTURAL path
+	// ("Item 8" → "Balance Sheet") rather than leaving the consumer to
+	// reverse-engineer one from the parser's chunk titles. Nil/empty when no
+	// TOC was persisted — the citation then simply omits title_path and the
+	// consumer falls back to its prior behaviour.
+	headingPaths := d.headingPathsForDoc(ctx, t)
+
 	for _, src := range sources {
 		sectionIDs := src.SectionIDs
 		if sectionIDs == nil {
@@ -321,6 +330,16 @@ func (d Deps) buildTreeWalkCitations(ctx context.Context, t *tree.Tree, res *ret
 			"section_ids": sectionIDs,
 		}
 
+		// Attach the heading path of the citation's primary (first, i.e.
+		// earliest-page) section. This mirrors how a consumer anchors a
+		// page-range citation to one structural location, and is exactly
+		// the field the bench's path-correctness metric reads.
+		if len(sectionIDs) > 0 {
+			if hp := headingPaths[sectionIDs[0]]; len(hp) > 0 {
+				c["title_path"] = hp
+			}
+		}
+
 		// Quote extraction is best-effort: an LLM blip or empty
 		// content returns no quote, which is a normal degradation
 		// path. We materialise the cited content from storage and
@@ -354,6 +373,33 @@ func (d Deps) buildTreeWalkCitations(ctx context.Context, t *tree.Tree, res *ret
 	return citations
 }
 
+// headingPathsForDoc resolves the canonical section-ID → heading-path
+// map for the document, reading the persisted LLM TOC tree through the
+// strategy's TOC provider and reconciling it with the section tree via
+// tree.BuildHeadingPaths (HAL-109).
+//
+// Every failure mode degrades to nil (no heading paths): no strategy /
+// provider wired, no TOC persisted (retrieval.ErrNoTOC), a fetch error,
+// or unparseable TOC JSON. A nil map is safe to index — citations then
+// omit title_path and the consumer falls back to its prior behaviour, so
+// this never makes a response worse than before the TOC was available.
+func (d Deps) headingPathsForDoc(ctx context.Context, t *tree.Tree) map[tree.SectionID][]string {
+	if t == nil || t.Root == nil || d.TreeWalkStrategy == nil || d.TreeWalkStrategy.TOC == nil {
+		return nil
+	}
+	raw, err := d.TreeWalkStrategy.TOC.GetTOC(ctx, t.DocumentID)
+	if err != nil || len(raw) == 0 {
+		return nil
+	}
+	var nodes []tree.TOCNode
+	if err := json.Unmarshal(raw, &nodes); err != nil {
+		d.Logger.Warn("answer/treewalk: TOC parse failed; citations omit heading paths",
+			"err", err, "document_id", t.DocumentID)
+		return nil
+	}
+	return tree.BuildHeadingPaths(t.Root, nodes)
+}
+
 // materialiseCitedContent loads + concatenates every cited
 // section's content. Used for answer-span extraction over the
 // pages the model relied on, so the quote can have real byte

internal/api/treewalk_citations_test.go

@@ -0,0 +1,107 @@
+package api
+
+import (
+	"context"
+	"encoding/json"
+	"log/slog"
+	"reflect"
+	"testing"
+
+	"github.com/hallelx2/vectorless-engine/pkg/retrieval"
+	"github.com/hallelx2/vectorless-engine/pkg/tree"
+)
+
+// stubTOCProvider returns canned TOC bytes (or an error) for the
+// citation builder's heading-path lookup.
+type stubTOCProvider struct {
+	raw []byte
+	err error
+}
+
+func (s stubTOCProvider) GetTOC(context.Context, tree.DocumentID) ([]byte, error) {
+	return s.raw, s.err
+}
+
+// citationTestTOC mirrors buildTreeWalkTestTree's page layout as a
+// logical outline: Setup{Install 1-2, Configuration 3-4},
+// Usage{Querying 5-7, Debt 8-9}.
+func citationTestTOC(t *testing.T) []byte {
+	t.Helper()
+	toc := []tree.TOCNode{
+		{Title: "Setup", StartPage: 1, EndPage: 4, Nodes: []tree.TOCNode{
+			{Title: "Install", StartPage: 1, EndPage: 2},
+			{Title: "Configuration", StartPage: 3, EndPage: 4},
+		}},
+		{Title: "Usage", StartPage: 5, EndPage: 9, Nodes: []tree.TOCNode{
+			{Title: "Querying", StartPage: 5, EndPage: 7},
+			{Title: "Debt", StartPage: 8, EndPage: 9},
+		}},
+	}
+	raw, err := json.Marshal(toc)
+	if err != nil {
+		t.Fatalf("marshal toc: %v", err)
+	}
+	return raw
+}
+
+// depsWithTOC builds a minimal Deps for buildTreeWalkCitations: no LLM
+// (so quote extraction is skipped) and a stub TOC provider on the
+// strategy.
+func depsWithTOC(toc []byte, tocErr error) Deps {
+	return Deps{
+		Logger: slog.Default(),
+		TreeWalkStrategy: &retrieval.TreeWalkStrategy{
+			TOC: stubTOCProvider{raw: toc, err: tocErr},
+		},
+	}
+}
+
+// TestBuildTreeWalkCitations_EmitsHeadingPath is the HAL-70 regression:
+// a citation must carry the canonical heading path of its primary
+// section, resolved from the TOC — the field the bench's
+// path_correct@1 metric reads.
+func TestBuildTreeWalkCitations_EmitsHeadingPath(t *testing.T) {
+	d := depsWithTOC(citationTestTOC(t), nil)
+	tr := buildTreeWalkTestTree()
+	res := &retrieval.Result{CitedPages: [][2]int{{1, 2}}}
+
+	cites := d.buildTreeWalkCitations(context.Background(), tr, res, "how do I install?", "")
+	if len(cites) != 1 {
+		t.Fatalf("want 1 citation, got %d: %v", len(cites), cites)
+	}
+	c := cites[0]
+
+	// Primary section for pages 1-2 is the leaf sec_a1 (Install).
+	ids, _ := c["section_ids"].([]tree.SectionID)
+	if len(ids) == 0 || ids[0] != "sec_a1" {
+		t.Fatalf("expected primary section sec_a1, got %v", c["section_ids"])
+	}
+
+	got, ok := c["title_path"].([]string)
+	if !ok {
+		t.Fatalf("citation is missing a title_path: %#v", c)
+	}
+	if want := []string{"Setup", "Install"}; !reflect.DeepEqual(got, want) {
+		t.Fatalf("title_path mismatch: got=%v want=%v", got, want)
+	}
+}
+
+// TestBuildTreeWalkCitations_NoTOCOmitsHeadingPath: when no TOC is
+// persisted (ErrNoTOC), the citation degrades gracefully — section_ids
+// and pages are still present, title_path is simply absent.
+func TestBuildTreeWalkCitations_NoTOCOmitsHeadingPath(t *testing.T) {
+	d := depsWithTOC(nil, retrieval.ErrNoTOC)
+	tr := buildTreeWalkTestTree()
+	res := &retrieval.Result{CitedPages: [][2]int{{5, 7}}}
+
+	cites := d.buildTreeWalkCitations(context.Background(), tr, res, "how do I query?", "")
+	if len(cites) != 1 {
+		t.Fatalf("want 1 citation, got %d", len(cites))
+	}
+	if _, present := cites[0]["title_path"]; present {
+		t.Fatalf("title_path must be absent without a TOC, got %v", cites[0]["title_path"])
+	}
+	if _, present := cites[0]["section_ids"]; !present {
+		t.Fatalf("section_ids must still be present")
+	}
+}