fix(wiki): normalise heading casing/whitespace before matching in Merge

Page-aware extraction (previous commit) asks the model to revise the
page it's given, but nothing guarantees it reproduces a heading's
exact on-disk text. Before this, Merge matched sections by exact
string equality, so "Key Claims" vs "Key claims" (or trailing
whitespace) silently created a parallel section instead of updating
the original — the drift #106 calls out by name.

Merge's idx map and the bulletSections/appendSections strategy lookup
now key on normalizeHeading (trim + lowercase), so casing/whitespace
drift matches the same section. The on-disk heading's original text
(secsA[i].heading) is untouched and is what gets written out — the
normalized form is used only to decide "is this the same section,"
never to rewrite a human's heading casing.

Touches wiki/merge.go, which is explicitly out of casual-edit scope
per today's review history on this file: justified because heading
normalization is #106's own checklist item and has to hook into
exactly this matching logic — there's nowhere else to put it. Scope
kept minimal: the two lookup sites only, no other behavior in this
file changed. bulletSectionsNorm/appendSectionsNorm exist because
matching a section by its normalized key but then classifying its
merge strategy by exact-string lookup would silently fall through to
"keep a's version" for a case-drifted bullet/append heading — dropping
b's content instead of unioning or appending it.

Closes hyperguild#106.

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

ingestion/internal/wiki/merge_test.go

@@ -305,6 +305,42 @@ Rewritten by the model.
 	assert.Equal(t, want, got)
 }
 
+// --- hyperguild#106: heading casing drift must not fork a section ----------
+// Page-aware extraction asks the model to revise the page it's given, but
+// nothing guarantees the model reproduces a heading's exact on-disk casing
+// ("Key Claims" vs "Key claims"). Without normalizing the match key, that
+// drift silently creates a near-duplicate section instead of updating the
+// original — the failure mode #106 exists to close.
+
+func TestMerge_HeadingCaseDriftMatchesSameSection(t *testing.T) {
+	a := Page{Path: "p.md", Content: "---\ntitle: X\n---\n\n## Key Claims\n\n- Appetite beats estimation.\n"}
+	b := Page{Path: "p.md", Content: "---\ntitle: X\n---\n\n## Key claims\n\n- New pattern holds under load.\n"}
+
+	got := Merge(a, b).Content
+
+	assert.Equal(t, 1, strings.Count(got, "## Key"),
+		"casing drift in the heading must not create a second section")
+	assert.Contains(t, got, "Appetite beats estimation.")
+	assert.Contains(t, got, "New pattern holds under load.",
+		"the differently-cased section's content must still be unioned in")
+	assert.Contains(t, got, "## Key Claims",
+		"the on-disk heading's original casing must be preserved in the output")
+	assert.NotContains(t, got, "## Key claims",
+		"the fresh extraction's heading casing must not overwrite the on-disk display text")
+}
+
+func TestMerge_HeadingWhitespaceDriftMatchesSameSection(t *testing.T) {
+	a := Page{Path: "p.md", Content: "---\ntitle: X\n---\n\n## Open Questions \n\n- Does appetite scale?\n"}
+	b := Page{Path: "p.md", Content: "---\ntitle: X\n---\n\n##  Open Questions\n\n- What about remote teams?\n"}
+
+	got := Merge(a, b).Content
+
+	assert.Equal(t, 1, strings.Count(got, "## Open Questions"),
+		"surrounding whitespace drift in the heading must not create a second section")
+	assert.Contains(t, got, "Does appetite scale?")
+	assert.Contains(t, got, "What about remote teams?")
+}
+
 func TestRefreshOwned_CommentLinesSurviveUnmangled(t *testing.T) {
 	// Third review round on PR #126: parseFrontmatterBlock dropped the
 	// sibling api package's comment guard (strings.HasPrefix(trimmed, "#")).