perf(vision): reuse OCR runtime sessions (#391)

* perf(vision): reuse OCR runtime sessions

* fix(vision): harden OCR engine caching

docs/rust-api.md

@@ -380,6 +380,19 @@ provides the same native-only behavior through the OCR result/provenance
 shape; `Force` renders every selected page. Learned layout intentionally
 returns an explicit unsupported error in this lightweight pipeline.
 
+The one-call API keeps the most recently used verified OCR engine in process.
+Long-lived workers therefore verify the pinned artifacts and build the ONNX
+sessions once, then reuse those loaded sessions across documents. The cache is
+bounded to one model configuration and keyed by normalized model/runtime paths
+plus the pinned manifest revision and artifact digests; switching the model
+directory, runtime library, or compiled manifest replaces it. An active engine
+owns the model data it already verified, so mutating artifacts in place does
+not hot-reload a running process; restart the process when intentionally
+replacing files at the same paths. CPU inference uses at most four intra-op
+threads per ONNX session so a single small page does not oversubscribe larger
+hosts, and recognizes variable-width line crops individually to avoid
+padding-heavy CPU batches.
+
 Build the CLI with the same opt-in feature:
 
 ```bash