feat: add 3 creative slides — tech decisions, request tracing, lessons learned

- why-not-x.md: Engineering decision rationale (Go vs Python, SQLite vs PG, etc.)
- request-journey.md: Full HTTP-to-pixel request tracing with timing breakdown
- lessons.md: 4 real engineering pitfalls and how they were solved
- Wire all 3 new pages into slides.md in logical positions

decks/gen2D/slides.md

@@ -57,6 +57,10 @@ src: ./pages/architecture.md
 src: ./pages/tech-stack.md
 ---
 
+---
+src: ./pages/why-not-x.md
+---
+
 ---
 section: '核心实现 — Eino 管线'
 src: ./pages/eino-graph.md
@@ -84,6 +88,10 @@ src: ./pages/async-deep-dive.md
 src: ./pages/dedup-ws.md
 ---
 
+---
+src: ./pages/request-journey.md
+---
+
 ---
 section: '核心实现 — 分布式限流'
 src: ./pages/ratelimit-overview.md
@@ -109,6 +117,10 @@ src: ./pages/harness.md
 
 ---
 section: '总结'
+src: ./pages/lessons.md
+---
+
+---
 src: ./pages/highlights.md
 ---