[Perf] Add reusable exact projection caching with MiniMax-H3 integration (#7987)

Signed-off-by: lishunyang12 <lishunyang12@users.noreply.github.com>
Signed-off-by: Armaan Amatya <armaanamatya2014@gmail.com>
Co-authored-by: lishunyang12 <lishunyang12@users.noreply.github.com>
Co-authored-by: Armaan Amatya <38102001+armaanamatya@users.noreply.github.com>

docs/design/module/diffusion/diffusion_model_integration.md

@@ -57,3 +57,58 @@ cross-stage routing.
 
 Test registry selection, checkpoint loading, minimal inference, input and output
 contracts, and every declared optional capability.
+
+## Exact conditioning projection reuse
+
+Models with repeated conditioning projections can use `ExactProjectionCache`
+from `vllm_omni.diffusion.cache`. It stores actual projection outputs for identical
+conditioning tensors and unchanged weights, with a byte budget and TP-wide hit
+coordination. It has no model names, sampling schedules, projection layouts, or
+artifact formats built in. MiniMax-H3 is its first model integration.
+
+Keep one cache per model instance and prepare each conditioning tensor once
+before its projections. For a leaf linear with fixed SiLU preprocessing:
+
+```python
+from torch.nn.functional import silu
+
+from vllm_omni.diffusion.cache import ExactProjectionCache
+
+cache = ExactProjectionCache(max_bytes=256 * 1024**2)
+
+# Inside an inference forward, with gradients disabled:
+cache.prepare(conditioning)
+modulation = cache.project(
+    "block.0.modulation",
+    projection,
+    conditioning,
+    lambda: projection(silu(conditioning)),
+)
+```
+
+The original computation must depend only on the supplied conditioning tensor,
+the leaf projection's weights/buffers, and fixed preprocessing. Include prompt,
+guidance, modality, or other request-dependent conditioning in the actual input;
+a timestep-only key is unsafe when those affect the result. Do not mutate the
+input between `prepare` and `project`, or modify outputs returned on cache hits.
+Use separate cache instances for concurrent forwards with different inputs.
+
+Call `clear()` when loading or switching adapters, changing projection behavior,
+or moving the model. Weight-version checks invalidate individual entries.
+Compilation, gradient-enabled execution, projection hooks, and unversioned
+inference weights use the original computation. Validate cold/warm parity,
+invalidation, and rank-local misses when integrating another model.
+Hook checks include global hooks and hooks on wrapped submodules. Numerical
+settings are captured at each projection call, so entering an autocast context
+after `prepare()` cannot reuse an output from a different precision.
+
+Validated offline results can be supplied through `_lookup_precomputed()`;
+this extension is consulted only at TP1 on runtime-cache misses. Artifact
+validation, weight binding, and model-specific layouts remain in the adapter.
+H3's adapter retains its main/Ref2VA sidecars and their eager-only admission.
+
+The cache retains projection results and leaves all original weights available.
+It does not reduce weight residency or cancel layerwise prefetch. A future
+integration that skips weight transfers must coordinate with `OffloadPlan`
+before prefetch and provide a safe miss path; moving weight storage currently
+causes conservative cache invalidation.