feat(ab-corpus): add deepseek-v4-pro client + route

DeepSeek wasn't in the harness; A/B v2 needed it for the 5-model run.
api.deepseek.com is OpenAI-compatible, so the client mirrors the
minimax pattern (callOpenAICompat with DEEPSEEK_API_KEY env, override-
able DEEPSEEK_BASE_URL).

Router: /^deepseek/i routes to the new client. Aliases `deepseek` /
`deepseek-pro` resolve to `deepseek-v4-pro` (current flagship per
docs); `deepseek-v4-flash` and the deprecated `deepseek-chat` /
`deepseek-reasoner` ids pass through verbatim until the 2026-07-24
sunset upstream.

Verified live with mobile-bio-textarea smoke (B emits batch_design,
T emits add_textarea_v0). Full ab-v2 results across all 5 models in
docs/notes/2026-04-28-ab-v2-results.md.

scripts/ab-corpus/clients/deepseek.ts

@@ -0,0 +1,42 @@
+/**
+ * DeepSeek wrapper. OpenAI-compatible endpoint at api.deepseek.com.
+ * Key in `DEEPSEEK_API_KEY`. Latest flagship at time of wiring is
+ * `deepseek-v4-pro`; `deepseek-v4-flash` is the lighter variant; the
+ * older `deepseek-chat` / `deepseek-reasoner` IDs are deprecated
+ * (sunset 2026-07-24) but the harness routes them anyway so corpus
+ * comparisons against pre-deprecation runs still work.
+ *
+ * Docs: https://api-docs.deepseek.com/zh-cn/
+ */
+
+import { callOpenAICompat } from './openai-compat';
+
+const DEEPSEEK_BASE_URL = process.env.DEEPSEEK_BASE_URL ?? 'https://api.deepseek.com';
+const DEEPSEEK_API_KEY_ENV = 'DEEPSEEK_API_KEY';
+
+export interface CallDeepSeekArgs {
+  model: string;
+  system: string;
+  user: string;
+  temperature?: number;
+  maxTokens?: number;
+}
+
+export async function callDeepSeek(args: CallDeepSeekArgs): Promise<string> {
+  const apiKey = process.env[DEEPSEEK_API_KEY_ENV];
+  if (!apiKey) {
+    throw new Error(
+      `${DEEPSEEK_API_KEY_ENV} not set — export it before running with --live, or pass --dry-run to use fixtures`,
+    );
+  }
+  return callOpenAICompat({
+    baseURL: DEEPSEEK_BASE_URL,
+    apiKey,
+    model: args.model,
+    system: args.system,
+    user: args.user,
+    temperature: args.temperature,
+    maxTokens: args.maxTokens,
+    label: 'deepseek',
+  });
+}

scripts/ab-corpus/real-model.ts

@@ -16,6 +16,7 @@ import { callMinimax } from './clients/minimax';
 import { callCodex } from './clients/codex-cli';
 import { callBailian } from './clients/bailian';
 import { callArk } from './clients/ark';
+import { callDeepSeek } from './clients/deepseek';
 import { buildSystemPrompt } from './build-prompt';
 import type { ModelCall } from './stub-model';
 
@@ -100,8 +101,20 @@ export async function realModelCall(call: ModelCall): Promise<string> {
       user,
     });
   }
+  // DeepSeek direct (api.deepseek.com, OpenAI-compat). Latest flagship
+  // is `deepseek-v4-pro`; `deepseek-v4-flash` is the lighter variant.
+  // The pre-2026-07-24 ids `deepseek-chat` / `deepseek-reasoner` are
+  // deprecated upstream but routed here too so older corpus runs stay
+  // reproducible until cutoff.
+  if (/^deepseek/i.test(model)) {
+    return callDeepSeek({
+      model: mapDeepSeekId(model),
+      system: built.system,
+      user,
+    });
+  }
   throw new Error(
-    `No live adapter for model "${model}". Supported: minimax-* (MINIMAX_API_KEY), gpt-*/o1/o3/o4 (Codex CLI), glm-5.1 / kimi-k2.6 (ARK_CODING_KEY), glm-5 / kimi-k2.5 (DASHSCOPE_BAILIAN_CODING_KEY).`,
+    `No live adapter for model "${model}". Supported: minimax-* (MINIMAX_API_KEY), gpt-*/o1/o3/o4 (Codex CLI), glm-5.1 / kimi-k2.6 (ARK_CODING_KEY), glm-5 / kimi-k2.5 (DASHSCOPE_BAILIAN_CODING_KEY), deepseek-* (DEEPSEEK_API_KEY).`,
   );
 }
 
@@ -175,3 +188,16 @@ function mapGlmBailianIdInternal(id: string): string {
   if (lower === 'glm-5' || lower === 'glm-4.7' || lower === 'glm-coding') return 'glm-4.7';
   return id;
 }
+
+function mapDeepSeekId(id: string): string {
+  // DeepSeek's current flagship is `deepseek-v4-pro`. Harness aliases
+  // `deepseek` (bare) and `deepseek-pro` resolve to that. The lighter
+  // `deepseek-v4-flash` and the deprecated `deepseek-chat` /
+  // `deepseek-reasoner` ids pass through verbatim — server still
+  // accepts them until 2026-07-24 sunset per docs.
+  const lower = id.toLowerCase();
+  if (lower === 'deepseek' || lower === 'deepseek-pro' || lower === 'deepseek-v4-pro') {
+    return 'deepseek-v4-pro';
+  }
+  return id;
+}