feat(docs): add Appendix glossary and page for undocumented tags

apps/docs/topic-landings.ts

@@ -4,16 +4,19 @@
  * tag", read by the sides that must agree:
  *
  * - `components/ProblemBadges.astro` links a covered tag's badge to its
- *   landing page, and every other tag's badge to its Appendix guide, `/appendix/<slug>`;
- * - `components/BacklinksList.astro` lists, under a landing or guide page,
- *   every problem whose badge links there — both via `tagTarget()`;
- * - `apps/cli/sync.ts` creates a placeholder `content/appendix/<slug>.mdx` for
- *   every tag a problem page carries that is NOT a key here.
+ *   landing page, and every other tag's badge to its Appendix card,
+ *   `/appendix#<slug>` (`appendix.ts`);
+ * - `components/BacklinksList.astro` lists, under a landing page, every
+ *   problem whose badge links there — both via `tagTarget()`;
+ * - `apps/cli/sync.ts` warns about a tag a problem page carries that is
+ *   neither a key here nor an Appendix term.
  *
- * Add a key when a campaign topic starts teaching a tag; its guide page is then
- * dead weight, delete it by hand. A tag naming two topics (DFS/BFS: tree and
+ * Add a key when a campaign topic starts teaching a tag; its Appendix term is
+ * then dead weight, delete it. A tag naming two topics (DFS/BFS: tree and
  * graph; DP: 1-D and grid) lists both; the badge picks per page.
  */
+import { APPENDIX } from "./appendix.ts";
+
 export const TOPIC_LANDINGS: Record<string, readonly string[]> = {
   "hash-table": ["/hash-table"],
   "prefix-sum": ["/prefix-sum"],
@@ -43,9 +46,9 @@ export interface TopicTag {
 }
 
 /**
- * Where a problem page's badge for `tag` points: a root-relative landing or
- * guide route (the caller adds the base), or LeetCode's problem list when the
- * site serves neither, so a badge never links to a 404.
+ * Where a problem page's badge for `tag` points: a root-relative landing
+ * route or Appendix anchor (the caller adds the base), or LeetCode's problem
+ * list when the site has neither, so a badge never links to a 404.
  *
  * A tag naming two landings links to the page's own topic — its route's first
  * segment — when that is one of them; otherwise DFS/BFS prefer the tree topic
@@ -55,13 +58,12 @@ export function tagTarget(
   tag: TopicTag,
   page: { route: string; tags: readonly TopicTag[] },
   served: ReadonlySet<string>,
-): { href: string; kind: "landing" | "guide" | "external" } {
+): { href: string; kind: "landing" | "appendix" | "external" } {
   const landings = (TOPIC_LANDINGS[tag.slug] ?? []).filter((path) => served.has(path));
   const own = `/${page.route.split("/")[1]}`;
   const prefer = page.tags.some((t) => t.slug === "tree") ? "/tree-" : "/graph-";
   const landing = landings.includes(own) ? own : (landings.find((p) => p.startsWith(prefer)) ?? landings[0]);
   if (landing) return { href: landing, kind: "landing" };
-  const guide = `/appendix/${tag.slug}`;
-  if (served.has(guide)) return { href: guide, kind: "guide" };
+  if (APPENDIX[tag.slug]) return { href: `/appendix#${tag.slug}`, kind: "appendix" };
   return { href: `https://leetcode.com/problem-list/${tag.slug}/`, kind: "external" };
 }