Add Achievements page to Game Node API section

docs/home/350-game-node-api/2-achievements.md

@@ -0,0 +1,98 @@
+# Achievements
+
+[PRC-1] defines a standard API for Paima Engine game nodes to serve achievement
+metadata and progress. Paima Engine itself includes an implementation of this
+API that only requires your game to export an achivement list and submit SQL
+updates to set player progress.
+
+## Achievement metadata
+
+When serving the achievements API, Paima Engine will attempt to use the `achievements` export with type `Promise<AchievementMetadata>` from your `packaged/endpoints.cjs`, if it exists. 
+
+```ts
+// api/src/index.ts
+const achievements: Promise<AchievementMetadata> = Promise.resolve({
+    game: {
+        id: "example",
+    },
+    list: [
+        {
+            name: "finish-chapter-1",
+            displayName: "Over The River",
+            description: "Finish Chapter 1.",
+        },
+        {
+            name: "finish-chapter-2",
+            displayName: "Through The Woods",
+            description: "Finish Chapter 2.",
+        },
+    ],
+});
+
+export default RegisterRoutes;  // The usual default export.
+export { achievements };  // Include this to enable the achievement API.
+```
+
+Achievement metadata uses types defined in [PRC-1] as well as the following:
+```ts
+/** The type of the `achievements` export of `endpoints.cjs`. */
+export interface AchievementMetadata {
+  /** Game ID, name, and version. */
+  game: Game;
+  /** Achievement types. */
+  list: Achievement[];
+  /**
+   * Per-language overrides for achievement display names and descriptions.
+   * Falls back to base definition whenever absent.
+   */
+  languages?: {
+    [language: string]: {
+      [name: string]: {
+        displayName?: string;
+        description?: string;
+      };
+    };
+  };
+}
+```
+
+TypeScript definitions for `AchievementMetadata` and [PRC-1] can be imported from `@paima/utils-backend`:
+```ts
+import type { AchievementMetadata, Game, Achievement } from '@paima/utils-backend';
+```
+
+## Achievement progress
+
+Paima SDK exports `pgtyped` queries to store and retrieve achievement progress.
+They can be imported and used in your API or a state transition function:
+
+```js
+import { getAchievementProgress, setAchievementProgress } from '@paima/db';
+// ... other imports ...
+
+async function wonBattle(wallet: number, blockTime: Date, dbConn: Pool): Promise<SQLUpdate[]> {
+  // Get user's current achievement progress.
+  const row = (await getAchievementProgress.run({ wallet, names: ['win-10-battles'] }, dbConn))[0];
+  if (!row?.completed_date) {
+    // Not complete yet. Add one, mark completed if needed, and store it back.
+    const newProgress = (row?.progress ?? 0) + 1;
+    return [
+      [setAchievementProgress, {
+        name: 'win-10-battles',
+        wallet,
+        completed_date: newProgress >= 10 ? blockTime : null,
+        progress: newProgress,
+        total: 10,
+      } satisfies ISetAchievementProgressParams],
+    ];
+  } else {
+    // Already complete. Nothing to update.
+    return [];
+  }
+}
+```
+
+Achievement progress stored this way will be served by Paima Engine's built-in
+PRC-1 implementation.
+
+[PRC-1]: ../20000-PRCs/prc-1.md

src/css/custom.css

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   @apply mb-3;
 }
 
+code {
+  vertical-align: unset;
+}
+
 .img-full {
   @apply w-full max-w-full;
 }
@@ -220,4 +224,4 @@ img {
 
 .code-block-alternate-color-line:nth-child(odd) {
   background-color: rgba(0, 0, 0, 0.05);
-}
+}