fix: split overflowing slides to fit frankfurt theme header/footer bars

- eino-deep-dive.md: split slide 2 into code-only + explanation+SVG slides
- async-deep-dive.md: split slide 2 into code-only + progress+SVG slides
- harness.md: split slide 2 (4 Items + SVG) into two slides (2 Items each)
- request-journey.md: split into middleware/handler + pipeline/finalization slides

The frankfurt theme header (~40px) overlaps ~7px into the content zone,
leaving ~467px effective height. Dense slides with code+Items+images
exceeded this budget and were hidden behind the bars.

decks/gen2D/pages/async-deep-dive.md

@@ -48,10 +48,6 @@ func (q *TaskQueue) Enqueue(job *TaskJob) {
 
 Context 注入 + 多通道推送
 
-<div class="grid grid-cols-2 gap-4 mt-2">
-
-<div>
-
 ```go {1-3|5-8|all}
 func WithProgressReporter(ctx context.Context, r ProgressReporter) context.Context {
     return context.WithValue(ctx, progressCtxKey, r)
@@ -62,13 +58,9 @@ reporter := GetProgressReporter(ctx)
 reporter.Report(Progress{Stage: "prompt", Percent: 25})
 ```
 
-<div class="flex justify-center mt-2">
-  <img src="../public/async-task.svg" class="w-full max-h-[160px] object-contain" />
-</div>
+---
 
-</div>
-
-<div class="flex flex-col justify-center">
+# 进度推送 — 阶段与通道
 
 <v-clicks>
 
@@ -82,6 +74,6 @@ reporter.Report(Progress{Stage: "prompt", Percent: 25})
 
 </v-clicks>
 
-</div>
-
+<div class="flex justify-center mt-2">
+  <img src="../public/async-task.svg" class="w-full max-h-[200px] object-contain" />
 </div>