feat: add heading anchors for direct linking to article sections

Uses markdown-it-anchor to generate linkable IDs on h2-h4 headings.
Headings become clickable links with a subtle # indicator on hover.
Includes scroll-margin-top so anchored headings don't hide behind
the sticky header.

package-lock.json

@@ -19,6 +19,7 @@
         "eleventy-plugin-embed-everything": "^1.21.0",
         "html-minifier-terser": "^7.0.0",
         "markdown-it": "^14.0.0",
+        "markdown-it-anchor": "^9.2.0",
         "pagefind": "^1.3.0",
         "rss-parser": "^3.13.0"
       },
@@ -1053,6 +1054,31 @@
         "tailwindcss": ">=3.0.0 || insiders || >=4.0.0-alpha.20 || >=4.0.0-beta.1"
       }
     },
+    "node_modules/@types/linkify-it": {
+      "version": "5.0.0",
+      "resolved": "https://registry.npmjs.org/@types/linkify-it/-/linkify-it-5.0.0.tgz",
+      "integrity": "sha512-sVDA58zAw4eWAffKOaQH5/5j3XeayukzDk+ewSsnv3p4yJEZHCCzMDiZM8e0OUrRvmpGZ85jf4yDHkHsgBNr9Q==",
+      "license": "MIT",
+      "peer": true
+    },
+    "node_modules/@types/markdown-it": {
+      "version": "14.1.2",
+      "resolved": "https://registry.npmjs.org/@types/markdown-it/-/markdown-it-14.1.2.tgz",
+      "integrity": "sha512-promo4eFwuiW+TfGxhi+0x3czqTYJkG8qB17ZUJiVF10Xm7NLVRSLUsfRTU/6h1e24VvRnXCx+hG7li58lkzog==",
+      "license": "MIT",
+      "peer": true,
+      "dependencies": {
+        "@types/linkify-it": "^5",
+        "@types/mdurl": "^2"
+      }
+    },
+    "node_modules/@types/mdurl": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/@types/mdurl/-/mdurl-2.0.0.tgz",
+      "integrity": "sha512-RGdgjQUZba5p6QEFAVx2OGb8rQDL/cPRG7GiedRzMcJ1tYnUANBncjbSB1NRGwbvjcPeikRABz2nshyPk1bhWg==",
+      "license": "MIT",
+      "peer": true
+    },
     "node_modules/@types/node": {
       "version": "14.18.63",
       "resolved": "https://registry.npmjs.org/@types/node/-/node-14.18.63.tgz",
@@ -2812,6 +2838,16 @@
         "markdown-it": "bin/markdown-it.mjs"
       }
     },
+    "node_modules/markdown-it-anchor": {
+      "version": "9.2.0",
+      "resolved": "https://registry.npmjs.org/markdown-it-anchor/-/markdown-it-anchor-9.2.0.tgz",
+      "integrity": "sha512-sa2ErMQ6kKOA4l31gLGYliFQrMKkqSO0ZJgGhDHKijPf0pNFM9vghjAh3gn26pS4JDRs7Iwa9S36gxm3vgZTzg==",
+      "license": "Unlicense",
+      "peerDependencies": {
+        "@types/markdown-it": "*",
+        "markdown-it": "*"
+      }
+    },
     "node_modules/markdown-it/node_modules/entities": {
       "version": "4.5.0",
       "resolved": "https://registry.npmjs.org/entities/-/entities-4.5.0.tgz",