Implement ESRC-WP-0002/0003/0004: HTML/MD ingest, metadata, recovery

Add ingestHtml and ingestMarkdown with ADR-0003 pageless offset semantics,
PDF intrinsic metadata extraction with caller-wins merge (WP-0003), and
citation recovery primitives including re-ingest reconcile, local quote
search, and pluggable discovery hooks (ADR-0004). Mark all three workplans
finished with contract tests (80 passing).

src/markdown/extract.ts

@@ -0,0 +1,53 @@
+/**
+ * Markdown text extraction → canonical text + gap-free offset map.
+ *
+ * Implements ADR-0003 for the `markdown-rendered` representation. Markup is
+ * stripped to plain text before `normalize()` is applied.
+ */
+
+import { normalize, type OffsetMap } from "@citation-evidence/engine/shared";
+import { buildWholeTextOffsetMap } from "../shared/offset-map";
+
+const FENCED_CODE = /```[\s\S]*?```/g;
+const INLINE_CODE = /`([^`]+)`/g;
+const IMAGE = /!\[([^\]]*)\]\([^)]+\)/g;
+const LINK = /\[([^\]]+)\]\([^)]+\)/g;
+const HEADING = /^#{1,6}\s+/gm;
+const BLOCKQUOTE = /^>\s?/gm;
+const LIST_MARKER = /^[\s]*[-*+]\s+/gm;
+const ORDERED_LIST = /^[\s]*\d+\.\s+/gm;
+const EMPHASIS = /(\*\*|__)(.*?)\1/g;
+const ITALIC = /(\*|_)([^*_]+)\1/g;
+const STRIKETHROUGH = /~~(.*?)~~/g;
+const HORIZONTAL_RULE = /^[-*_]{3,}\s*$/gm;
+
+export interface MarkdownExtractionResult {
+  readonly canonicalText: string;
+  readonly offsetMap: OffsetMap;
+}
+
+export function extractMarkdown(source: string): MarkdownExtractionResult {
+  const rawText = markdownToPlainText(source);
+  const canonicalText = normalize(rawText).text;
+  return {
+    canonicalText,
+    offsetMap: buildWholeTextOffsetMap(canonicalText),
+  };
+}
+
+function markdownToPlainText(source: string): string {
+  let text = source;
+  text = text.replace(FENCED_CODE, "\n\n");
+  text = text.replace(INLINE_CODE, "$1");
+  text = text.replace(IMAGE, "$1");
+  text = text.replace(LINK, "$1");
+  text = text.replace(HEADING, "");
+  text = text.replace(BLOCKQUOTE, "");
+  text = text.replace(LIST_MARKER, "");
+  text = text.replace(ORDERED_LIST, "");
+  text = text.replace(EMPHASIS, "$2");
+  text = text.replace(ITALIC, "$2");
+  text = text.replace(STRIKETHROUGH, "$1");
+  text = text.replace(HORIZONTAL_RULE, "\n\n");
+  return text;
+}