p1-2: fixtures + snapshot tests for frontmatter parser

Two markdown fixtures with hand-authored JSON baselines that pin the
§0 Q9 derive output across runs:

- frontmatter-only.md exercises the YAML happy path with most fields,
  unknown keys, an `id:` field, and a non-UTC created_at (so the
  baseline shows original_timestamps preservation).
- mixed-lang.md is body-only with no `lang:` field; baseline pins the
  lingua autodetect result for our enabled language set.

A separate `emit_snapshots` test (marked `#[ignore]`) regenerates the
baselines from the current parser output. A determinism test parses
the fixture twice and asserts equality so any non-determinism (e.g.
key ordering, lingua nondeterminism) fails fast.

fixtures/markdown/frontmatter-only.md

@@ -0,0 +1,22 @@
+---
+title: Frontmatter Only
+aliases:
+  - fm-only
+  - first-fixture
+tags:
+  - parse
+  - test
+lang: en
+created_at: 2024-01-15T10:00:00+09:00
+updated_at: 2024-02-20T08:30:00Z
+source_type: note
+trust_level: secondary
+id: my-stable-handle
+custom_field: hello
+nested_obj:
+  key: value
+---
+
+# Body Heading
+
+Body paragraph.

fixtures/markdown/frontmatter-only.snapshot.json

@@ -0,0 +1,31 @@
+{
+  "metadata": {
+    "aliases": [
+      "fm-only",
+      "first-fixture"
+    ],
+    "tags": [
+      "parse",
+      "test"
+    ],
+    "created_at": "2024-01-15T01:00:00Z",
+    "updated_at": "2024-02-20T08:30:00Z",
+    "source_type": "note",
+    "trust_level": "secondary",
+    "user_id_alias": "my-stable-handle",
+    "user": {
+      "custom_field": "hello",
+      "lang": "en",
+      "nested_obj": {
+        "key": "value"
+      },
+      "original_timestamps": {
+        "created_at": "2024-01-15T10:00:00+09:00"
+      },
+      "title": "Frontmatter Only",
+      "user_id_alias": "my-stable-handle"
+    }
+  },
+  "span_present": true,
+  "warnings": []
+}

fixtures/markdown/mixed-lang.md

@@ -0,0 +1,9 @@
+# Mixed Language Note
+
+이 문서는 한국어와 영어가 섞여 있습니다. The body has both Korean
+sentences and English sentences. lingua는 통계적 언어 감지기를 제공합니다.
+This is to test that auto-detect picks one of `ko` or `en` deterministically
+when no `lang:` field is present in the frontmatter.
+
+본문은 첫 4 KB만 분석되지만, 짧은 문서에서도 잘 동작해야 합니다.
+The detector should pick the dominant language across the sample window.

fixtures/markdown/mixed-lang.snapshot.json

@@ -0,0 +1,16 @@
+{
+  "metadata": {
+    "aliases": [],
+    "tags": [],
+    "created_at": "2024-01-01T00:00:00Z",
+    "updated_at": "2024-01-02T00:00:00Z",
+    "source_type": "markdown",
+    "trust_level": "primary",
+    "user_id_alias": null,
+    "user": {
+      "lang": "en"
+    }
+  },
+  "span_present": false,
+  "warnings": []
+}