feat(engine): merge cloudflare-migration — paradigm D engine, BYOK proxy, story persistence (#95)

Squash-merge the cloudflare-migration branch (7 commits by Kai ki) into
staging with conflict resolution, feature integration, and bug fixes.

Engine:
- Paradigm D: single-stream Writer replacing dual-phase Plan/Beats
- Delete Architect agent; story bible generated via Writer <plan> tag
- Modular prompt architecture (segments/registry/builder)
- StreamRouter for tagged stream splitting (<plan>/<story>/<choices>)

Infrastructure:
- Cloudflare Workers deployment (wrangler.jsonc, OpenNext adapter)
- D1 database schema + Drizzle ORM (scaffolded, not yet active)
- R2 storage helpers (scaffolded, not yet active)
- Story persistence API routes + client-side persistence

BYOK (Bring Your Own Key):
- /api/llm/user-proxy with SSRF-protected LLM proxy (+ requireUser auth)
- CORS-aware fetch in ai-client: auto-detect CORS failure, fallback to
  server proxy transparently via OpenAI SDK custom fetch
- BYO config support added to classify-freeform and vision routes
- SettingsModal CORS privacy notice (keys never logged/stored)

SSE streaming:
- engineClient.ts: fetchSSE helper for progressive scene events
- startSession/requestScene accept optional emit callback
- Fix SSE error event field name (error → message) in scene/start routes

i18n integration:
- Wire buildLanguageDirective into paradigm D's prompt builder
- Update corsNotice i18n keys (zh-CN/en/ja) with CORS proxy privacy text
- Preserve Session.language + LanguageSwitcher from i18n commit

Co-authored-by: Kai ki <155355644+zbf1009@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

lib/engine/prompts/builder.ts

@@ -0,0 +1,59 @@
+import type { ChatMessage } from "@infiplot/ai-client";
+import type { Session } from "@infiplot/types";
+import { WRITER_SEGMENTS } from "./registry";
+import { buildWriterContext } from "../context";
+import { buildLanguageDirective } from "../prompts";
+
+/**
+ * Build the full ChatMessage[] for the Writer agent.
+ *
+ * Segments from the registry provide the system prompt (stable zone).
+ * ContextProvider supplies session-specific data (stable + dynamic zones).
+ * Dynamic parts are wrapped in a user message (Plan C: pseudo-dialogue closure).
+ */
+export function buildWriterStreamMessages(session: Session): ChatMessage[] {
+  const systemParts: string[] = [];
+
+  const segments = WRITER_SEGMENTS
+    .filter((s) => s.enabled)
+    .sort((a, b) => {
+      if (a.zone !== b.zone) return a.zone === "stable" ? -1 : 1;
+      return a.order - b.order;
+    });
+
+  for (const seg of segments) {
+    try {
+      const content =
+        typeof seg.content === "string" ? seg.content : seg.content(session);
+      if (content.trim()) systemParts.push(content);
+    } catch (err) {
+      console.warn(`[PromptBuilder] segment "${seg.id}" render failed, skipped:`, err);
+    }
+  }
+
+  const { stableParts, dynamicParts } = buildWriterContext(session);
+
+  const messages: ChatMessage[] = [];
+
+  // System message: segment content + stable context data
+  const systemContent = [
+    ...systemParts,
+    ...stableParts.filter((p) => p.trim()),
+  ].join("\n\n");
+
+  if (systemContent.trim()) {
+    messages.push({ role: "system", content: systemContent });
+  }
+
+  // User message: dynamic context data + pseudo-dialogue closure (Plan C)
+  const dynamicContent = dynamicParts.filter((p) => p.trim()).join("\n\n");
+  if (dynamicContent.trim()) {
+    const langDirective = buildLanguageDirective(session.language);
+    messages.push({
+      role: "user",
+      content: `编剧，下面是当前情境：\n\n${dynamicContent}\n\n现在请按上述指导开始创作，严格按 <plan>→<story>→<choices> 三段输出：<plan> 用 JSON 规划，<story> 写连贯散文正文，<choices> 给出选项。${langDirective}`,
+    });
+  }
+
+  return messages;
+}