docs: add teaching examples and video project

show/experiments/README.md

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+# 主线之外的实验
+
+`flattened_position.cuh` 保存此前把 `(seed,x,z)` 展平的实现。
+它已从 `cuda_steps.cu` 和默认构建移除，不再作为 03 或 Random 优化的基础。
+主线只讲按 seed → 固定坐标后遍历 seed → 复用轴项。
+旧测速中的 mode 31 与 03b 标签属于历史实验，见 `../results/mapping_diagnostics.json`。

show/experiments/flattened_position.cuh

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+#pragma once
+
+// Archived experiment; excluded from the teaching sequence and normal build.
+// Stage 03b (mode 31): flatten the whole (seed,x,z) product into fine work items.
+template<int stage> __global__ void positionKernel(
+    DeviceArgs a, show::Hit* hits, unsigned long long* count
+) {
+    const uint64_t area=a.width*a.height, total=a.seeds*area;
+    const uint64_t stride=uint64_t(gridDim.x)*blockDim.x;
+    for(uint64_t i=uint64_t(blockIdx.x)*blockDim.x+threadIdx.x;i<total;i+=stride) {
+        const uint64_t s=i/area, p=i%area;
+        const int32_t x=int32_t(int64_t(a.x0)+int64_t(p/a.height));
+        const int32_t z=int32_t(int64_t(a.z0)+int64_t(p%a.height));
+        if(show::solid<stage>(a.seed+s,x,z,a.n)) emit(a,hits,count,a.seed+s,x,z);
+    }
+}