fix(wiki): pair duplicate-heading occurrences positionally, not all to one index

Review of PR #132 found a real bug in the first-occurrence-wins fix:
routing every one of b's occurrences of a normalized heading to a's
single first occurrence means a self-merge (or a revision pass echoing
a page back unchanged, exactly what #106 can produce) pairs b's SECOND
"## NOTES" against a's FIRST "## Notes" -- different content, same
normalized key -- and MergeReport wrongly reports a disagreement where
none exists. Inert in #130 (fed the vestigial Merge wrapper, which
discards conflicts) and only became visible once correctly wired into
MergeReport, the function production actually calls.

Fixed by pairing occurrences positionally: a's on-disk sections sharing
a normalized heading are collected into an ordered list per key: b's
1st occurrence of that key pairs with a's 1st, b's 2nd with a's 2nd,
and so on. b having MORE occurrences than a appends the extra as new
content and registers it, so a third occurrence in b pairs with that
newly appended one rather than creating yet another duplicate.

Flip-tested: forcing every pairing back to position 0 (the prior fix's
actual behavior) turns the new self-merge test red; the positional
version stays green, and the original PR #130 regression test (fresh
content must route to the first occurrence when b has only one
occurrence) still passes unchanged -- the two fixes aren't in tension,
positional pairing subsumes first-occurrence-wins as the pos==0 case.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ingestion/internal/wiki/merge_test.go

@@ -367,6 +367,36 @@ func TestMerge_PreExistingCaseDuplicateHeadingsRouteToTheFirstOccurrence(t *test
 		"the second duplicate is untouched by this update, not merged into or overwritten")
 }
 
+func TestMergeReport_SelfMergeOfPreExistingDuplicateHeadingsReportsNoConflict(t *testing.T) {
+	// Found in review of PR #132: first-occurrence-wins routes ALL of b's
+	// occurrences of a normalized heading to a's FIRST occurrence, not to
+	// their own positional counterpart. So merging a page against itself
+	// (or an echoed-back copy, exactly what #106's revision prompt can
+	// produce) pairs b's second "## NOTES" against a's first "## Notes" —
+	// different content, same normalized key — and MergeReport wrongly
+	// reports a disagreement where none exists. Fixed by pairing
+	// positionally: b's Nth occurrence of a normalized heading matches a's
+	// Nth occurrence, not always the first.
+	page := Page{Path: "p.md", Content: "---\ntitle: X\n---\n\n## Notes\n\nFirst note.\n\n## NOTES\n\nSecond note.\n"}
+
+	_, conflicts := MergeReport(page, page)
+	assert.Empty(t, conflicts, "merging a page with pre-existing duplicate headings against itself must report no conflicts")
+}
+
+func TestMergeReport_DuplicateHeadingsPairPositionallyNotAllToFirst(t *testing.T) {
+	// Companion case: b has genuinely different content in its second
+	// occurrence of the normalized heading — that must compare against a's
+	// second occurrence (and report if they disagree), not against a's
+	// first (which would either false-positive if a's first differs, or
+	// false-negative-mask a real disagreement in the second pair).
+	a := Page{Path: "p.md", Content: "---\ntitle: X\n---\n\n## Notes\n\nFirst note.\n\n## NOTES\n\nSecond note.\n"}
+	b := Page{Path: "p.md", Content: "---\ntitle: X\n---\n\n## Notes\n\nFirst note.\n\n## NOTES\n\nSecond note, revised.\n"}
+
+	_, conflicts := MergeReport(a, b)
+	assert.Equal(t, []string{"NOTES"}, conflicts,
+		"only the second occurrence, which actually disagrees, is reported")
+}
+
 func TestRefreshOwned_CommentLinesSurviveUnmangled(t *testing.T) {
 	// Third review round on PR #126: parseFrontmatterBlock dropped the
 	// sibling api package's comment guard (strings.HasPrefix(trimmed, "#")).