fix(wiki): rewrite refreshOwned on a parsed line list, not fence-index tracking

Second review round on PR #126 found two more problems with the first
append fix:

1. When a has no frontmatter fence at all, the fallback spliced raw,
   unfenced "key: value" text straight into the body -- worse than the
   original bug, since the next parseSections call wouldn't recognize
   it as frontmatter and would absorb it as preamble text, duplicating
   on every subsequent merge.
2. The regression test only asserted substrings (Contains), which
   doesn't prove structural position. Two independent mutations
   (closeIdx locking onto the wrong fence, a reversed sort) both
   survived the full suite undetected.

Rewritten from string-index surgery (tracking a fence's position across
two passes over split lines) to a small parsed representation: fmLine
(key/value or raw), parseFrontmatterBlock, setLine (update in place or
append -- the one behavior the original code got wrong), and
renderFrontmatterBlock. Same shape as the sibling api package's own
frontmatter type, reproduced locally rather than imported (it's
unexported there, and this package's needs are smaller) because that
set()-appends-if-absent behavior was already proven correct by that
package's existing test suite -- this bug class shouldn't have existed
here in the first place.

A page with no fence gets one created to hold the new fields, instead
of corrupting the body. Tests now assert exact frontmatter block output
via assert.Equal, not Contains, and a new fixture (mixed replace+append
in one call) covers both code paths together.

Flip-tested: reversing the sort turns two of the new exact-match tests
red (the Contains-based version of the same test missed this on the
prior commit). The "wrong fence" bug class is now structurally
impossible rather than merely untested -- there's no fence-position
index left to get wrong.

Also fixed a byte-stability regression the new render introduced and
caught via the full pipeline suite: a key with no inline value (the
YAML block-list shape, "aliases:" followed by indented "- item" lines)
rendered as "aliases: " (trailing space) instead of "aliases:".

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

ingestion/internal/wiki/merge_test.go

@@ -244,9 +244,63 @@ Rewritten by the model.
 
 	got := Merge(existing, fresh).Content
 
-	assert.Contains(t, got, "source_hash: bbbb", "a generator-owned field a page never had before must be appended, not dropped")
-	assert.Contains(t, got, "last_updated: 2026-08-22")
-	assert.Contains(t, got, "title: 'X'", "a human-set title must still survive regeneration")
-	assert.Contains(t, got, "Original.")
-	assert.NotContains(t, got, "Renamed by the model")
+	// Exact frontmatter block, not Contains: the second review round found
+	// that a substring check doesn't prove the fields landed inside a valid
+	// fence, in the right order, or that the fence itself wasn't corrupted —
+	// two independent mutations (closeIdx targeting the wrong fence, reversed
+	// sort) both survived a Contains-only version of this test.
+	want := "---\ntitle: 'X'\nlast_updated: 2026-08-22\nsource_hash: bbbb\n---\n\n## Summary\n\nOriginal.\n"
+	assert.Equal(t, want, got)
+}
+
+func TestMerge_GeneratorOwnedFieldsCreateAFrontmatterBlockWhenThePageHadNoneAtAll(t *testing.T) {
+	// Second review round on PR #126: when a has no frontmatter fence at all,
+	// the append fallback spliced raw, unfenced "key: value" lines straight
+	// into the body -- worse than the original bug, because the next
+	// parseSections call wouldn't recognize them as frontmatter (they don't
+	// start with `---`), so they'd be absorbed as ordinary preamble text and
+	// re-duplicate on every subsequent merge instead of updating in place.
+	existing := Page{Path: "wiki/sources/x.md", Content: "## Summary\n\nOriginal.\n"}
+	fresh := Page{Path: "wiki/sources/x.md", Content: `---
+last_updated: 2026-08-22
+source_hash: bbbb
+---
+
+## Summary
+
+Rewritten by the model.
+`}
+
+	got := Merge(existing, fresh).Content
+
+	want := "---\nlast_updated: 2026-08-22\nsource_hash: bbbb\n---\n\n## Summary\n\nOriginal.\n"
+	assert.Equal(t, want, got)
+}
+
+func TestMerge_GeneratorOwnedFieldsMixedReplaceAndAppend(t *testing.T) {
+	// a already has last_updated (replace in place) but not source_hash
+	// (append) -- the two code paths in the same call, together.
+	existing := Page{Path: "wiki/sources/x.md", Content: `---
+title: 'X'
+last_updated: 2026-01-01
+---
+
+## Summary
+
+Original.
+`}
+	fresh := Page{Path: "wiki/sources/x.md", Content: `---
+last_updated: 2026-08-22
+source_hash: bbbb
+---
+
+## Summary
+
+Rewritten by the model.
+`}
+
+	got := Merge(existing, fresh).Content
+
+	want := "---\ntitle: 'X'\nlast_updated: 2026-08-22\nsource_hash: bbbb\n---\n\n## Summary\n\nOriginal.\n"
+	assert.Equal(t, want, got)
 }