perf: cache CPU position embeddings across DiT runners (#2211)

docs/pe_cache.md

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+# Position embedding cache
+
+`sd::PECache` stores CPU-side `sd::Tensor<float>` data. Each model runner owns
+its cache and calls the existing `make_input()` on every graph build. Cache
+hits skip PE generation; allocation and upload remain ordinary graph-input
+operations. `GGMLRunner` has no PE-specific code or resource hooks.
+
+The cache keeps the most recent key and tensor for each named PE input. A
+matching key returns the existing tensor by reference. Changing its parameters
+replaces that input's cached result. Different named inputs have independent
+storage. Alternating two layouts under the same name recomputes their PE;
+there is no LRU, device-buffer cache, memory budget, or graph lifecycle callback.
+CPU data lives with the model runner, including across `runner_end()` calls,
+and is released when its named entry is replaced or the cache is destroyed.
+
+## Adding a model
+
+Include `core/pe_cache.h` and declare a member:
+
+```cpp
+sd::PECache pe_cache_;
+```
+
+Construct the key before generating PE and place the generation code in the
+lazy callback:
+
+```cpp
+sd::PECacheKey key("model.image_pe");
+key.add(height).add(width).add(head_dim).add(theta)
+    .add(circular_x_enabled).add(circular_y_enabled);
+auto pe = make_input(pe_cache_.get_or_create(key, [&]() {
+    auto values = generate_position_embedding(/* existing arguments */);
+    return sd::Tensor<float>({2, 2, head_dim / 2, token_count}, std::move(values));
+}));
+```
+
+Keys must cover every variable affecting PE values or shape: geometry, text
+length, frequency settings, reference-image ordering and offsets, circular
+settings, and video/audio positions or frame rates as applicable.
+`add_shape()` records all four ggml dimensions, including trailing singleton
+dimensions. `add_shapes()` records an ordered list of reference shapes.
+`add(sd::Tensor<T>)` records both shape and values for explicit position inputs.
+Keys compare their complete encoded fields rather than a data pointer or hash.
+
+Use distinct names for PE inputs that coexist in a graph, such as Anima's
+adapter query and key PE. `make_input()` defers reading the CPU data until graph
+execution: do not replace a named entry while an unfinished graph references
+it. Adding or updating other named entries does not invalidate its storage.
+Return tensor shapes with the contiguous dimension first, and preserve each
+model's existing F32 values, layout, and final transformations.
+
+## Current integrations
+
+The cache is used by MiniT2I, the Flux runner and its variants, Flux3, Qwen Image,
+Qwen Image 2.1, Z-Image and L2P, Anima, Boogu, ERNIE Image, Fibo2, Ideogram4,
+Iris, Krea2, Lens, LLaDA Image, MAGE Flow, Ming Image, MOSS Image, PiD, PixArt,
+Wan, Hunyuan Video, LingBot Video, and LTX audio/video.
+
+Position embeddings stored as model weights and graph-side RoPE operations
+remain on their existing paths, including MiniMax H3 and SenseNova U1.