Plan 61: add bad fixtures for out-of-order, wrong-level, extra-section

Integration fixtures pin the exact diagnostic strings and line
numbers for the three new/changed behaviours:

- bad/out-of-order.md — doc lists ## Tasks before ## Goal; schema
  lists ## Goal before ## Tasks; expects a single "section
  \"## Tasks\" out of order: expected after \"## Goal\"".
- bad/wrong-level.md — ### Goal under schema ## Goal; expects
  "heading level mismatch for \"Goal\": expected h2, got h3".
- bad/extra-section.md — ## Extra sits between required ## Goal
  and ## Tasks; expects "unexpected section \"## Extra\"
  (expected \"## Goal\")".

Author: claude

internal/rules/MDS020-required-structure/bad/extra-section.md

@@ -0,0 +1,21 @@
+---
+settings:
+  schema: "../../internal/rules/MDS020-required-structure/bad/data/tmpl.md"
+diagnostics:
+  - line: 3
+    column: 1
+    message: 'unexpected section "## Extra" (expected "## Goal")'
+---
+# My Plan
+
+## Extra
+
+Extra description.
+
+## Goal
+
+Goal description.
+
+## Tasks
+
+Tasks description.

internal/rules/MDS020-required-structure/bad/out-of-order.md

@@ -0,0 +1,17 @@
+---
+settings:
+  schema: "../../internal/rules/MDS020-required-structure/bad/data/tmpl.md"
+diagnostics:
+  - line: 3
+    column: 1
+    message: 'section "## Tasks" out of order: expected after "## Goal"'
+---
+# My Plan
+
+## Tasks
+
+Tasks description.
+
+## Goal
+
+Goal description.

internal/rules/MDS020-required-structure/bad/wrong-level.md

@@ -0,0 +1,17 @@
+---
+settings:
+  schema: "../../internal/rules/MDS020-required-structure/bad/data/tmpl.md"
+diagnostics:
+  - line: 3
+    column: 1
+    message: 'heading level mismatch for "Goal": expected h2, got h3'
+---
+# My Plan
+
+### Goal
+
+Goal description.
+
+## Tasks
+
+Tasks description.