p1-3: snapshot tests + clippy fix

Adds two snapshot tests (`nested-headings.md`, `code-and-table.md`) under
crates/kb-parse-md/tests/blocks_snapshots.rs, with matching baseline JSON
next to each fixture. The snapshot view projects `kb_core::Inline` to
flat strings — `Inline` carries `serde(tag = "kind")` which is
incompatible with newtype variants holding a primitive (`Text(String)`),
so direct serialization of `ParsedBlock` would fail today. The view
preserves the contract that matters for P1-3 (heading paths, source
spans, payload kinds, payload text/code/table content) and will keep
working once kb-core fixes the Inline schema in a later task.

Also tightens `level_to_use >= 1 && <= 6` into `(1..=6).contains(&_)` to
satisfy `clippy::manual_range_contains`.

fixtures/markdown/code-and-table.blocks.snapshot.json

@@ -0,0 +1,63 @@
+{
+  "blocks": [
+    {
+      "kind": "heading",
+      "heading_path": [],
+      "source_span": {
+        "kind": "line",
+        "start": 1,
+        "end": 1
+      },
+      "payload": {
+        "kind": "heading",
+        "level": 1,
+        "text": "Code And Table"
+      }
+    },
+    {
+      "kind": "code",
+      "heading_path": [
+        "Code And Table"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 3,
+        "end": 7
+      },
+      "payload": {
+        "kind": "code",
+        "lang": "rust",
+        "code": "fn main() {\n    println!(\"hi\");\n}"
+      }
+    },
+    {
+      "kind": "table",
+      "heading_path": [
+        "Code And Table"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 9,
+        "end": 12
+      },
+      "payload": {
+        "kind": "table",
+        "headers": [
+          "col a",
+          "col b"
+        ],
+        "rows": [
+          [
+            "1",
+            "2"
+          ],
+          [
+            "3",
+            "4"
+          ]
+        ]
+      }
+    }
+  ],
+  "warnings": []
+}

fixtures/markdown/nested-headings.blocks.snapshot.json

@@ -0,0 +1,136 @@
+{
+  "blocks": [
+    {
+      "kind": "heading",
+      "heading_path": [],
+      "source_span": {
+        "kind": "line",
+        "start": 1,
+        "end": 1
+      },
+      "payload": {
+        "kind": "heading",
+        "level": 1,
+        "text": "Top"
+      }
+    },
+    {
+      "kind": "paragraph",
+      "heading_path": [
+        "Top"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 3,
+        "end": 3
+      },
+      "payload": {
+        "kind": "paragraph",
+        "text": "intro",
+        "inlines_flat": "intro"
+      }
+    },
+    {
+      "kind": "heading",
+      "heading_path": [
+        "Top"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 5,
+        "end": 5
+      },
+      "payload": {
+        "kind": "heading",
+        "level": 2,
+        "text": "Section A"
+      }
+    },
+    {
+      "kind": "paragraph",
+      "heading_path": [
+        "Top",
+        "Section A"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 7,
+        "end": 7
+      },
+      "payload": {
+        "kind": "paragraph",
+        "text": "body of A",
+        "inlines_flat": "body of A"
+      }
+    },
+    {
+      "kind": "heading",
+      "heading_path": [
+        "Top",
+        "Section A"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 9,
+        "end": 9
+      },
+      "payload": {
+        "kind": "heading",
+        "level": 3,
+        "text": "Sub A.1"
+      }
+    },
+    {
+      "kind": "paragraph",
+      "heading_path": [
+        "Top",
+        "Section A",
+        "Sub A.1"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 11,
+        "end": 11
+      },
+      "payload": {
+        "kind": "paragraph",
+        "text": "deeper",
+        "inlines_flat": "deeper"
+      }
+    },
+    {
+      "kind": "heading",
+      "heading_path": [
+        "Top"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 13,
+        "end": 13
+      },
+      "payload": {
+        "kind": "heading",
+        "level": 2,
+        "text": "Section B"
+      }
+    },
+    {
+      "kind": "paragraph",
+      "heading_path": [
+        "Top",
+        "Section B"
+      ],
+      "source_span": {
+        "kind": "line",
+        "start": 15,
+        "end": 15
+      },
+      "payload": {
+        "kind": "paragraph",
+        "text": "body of B",
+        "inlines_flat": "body of B"
+      }
+    }
+  ],
+  "warnings": []
+}