scripts: extract shared cordis-walk and md-fences helpers (jscpd gate)

Master's new duplication gate flagged 4 clones, all from the website scripts
reusing existing gate patterns by copy. Extracted the shared halves instead of
ignoring:
- scripts/cordis-walk.ts — cordisModuleBody / eventMembers / serviceClasses,
  one AST walk consumed by both gen-cordis-catalog and gen-website-api (the
  two renderers must agree on WHAT exists; now they structurally do).
- scripts/md-fences.ts — fence extractor with per-gate info-string
  classification, consumed by doc-typecheck and verify-website-yaml.

Behavior-neutral: regenerated catalogs and website pages are byte-identical
(--check green), doc-typecheck/yaml block counts unchanged. jscpd now reports
0 clones; full pre-push run 34/34 green.

scripts/md-fences.ts

@@ -0,0 +1,55 @@
+/**
+ * Shared fenced-code-block extractor for the Markdown doc gates
+ * (`doc-typecheck.ts`, `verify-website-yaml.ts`). One scanner, per-gate
+ * classification: each gate maps a fence info string (` ```ts `,
+ * ` ```yaml ignore-check `, …) to its own kind tag and receives every
+ * classified block with its 1-based opening-fence line.
+ */
+
+import { readFileSync } from 'node:fs'
+
+/** One extracted fenced block, classified by the caller's `classify`. */
+export interface Fence<K> {
+  /** 1-based line of the opening fence. */
+  line: number
+  kind: K
+  code: string
+}
+
+/**
+ * Extract every fenced block of `absPath` whose info string `classify` maps
+ * to a kind. Blocks classified `null` are skipped (their bodies are still
+ * consumed, so an unrelated fence can never leak into a tracked one).
+ *
+ * @param absPath — absolute path of the Markdown file.
+ * @param classify — info string (trimmed, e.g. `ts ignore-check`) → kind, or
+ *   null for fences this gate does not track.
+ * @returns the classified blocks in document order.
+ */
+export function extractFences<K>(absPath: string, classify: (info: string) => K | null): Fence<K>[] {
+  const lines = readFileSync(absPath, 'utf8').split('\n')
+  const blocks: Fence<K>[] = []
+  let open: { line: number; kind: K; body: string[] } | null = null
+  let skipping = false
+
+  lines.forEach((raw, i) => {
+    const fence = /^```(\s*)(\S.*)?$/.exec(raw)
+    if (!fence) {
+      if (open) open.body.push(raw)
+      return
+    }
+    if (open) {
+      blocks.push({ line: open.line, kind: open.kind, code: open.body.join('\n') })
+      open = null
+      return
+    }
+    if (skipping) {
+      skipping = false
+      return
+    }
+    const kind = classify((fence[2] ?? '').trim())
+    if (kind !== null) open = { line: i + 1, kind, body: [] }
+    else skipping = true
+  })
+  return blocks
+}