feat(vision): add OCR contracts (#355)

* feat(vision): add OCR contracts

* fix(vision): harden OCR contracts

* refactor(vision): use OCR terminology

docs/rust-api.md

@@ -117,12 +117,53 @@ let bytes = std::fs::read("document.pdf")?;
 let result = process_pdf_mem(&bytes)?;
 ```
 
+### Vision extension contracts
+
+The native-only `vision` feature exposes the stable seam used by OCR
+integrations without selecting or embedding an inference runtime. The
+separate `model-cache` feature adds pinned artifact management:
+
+- `PageRenderer`, `OcrEngine`, and `LayoutEngine` traits;
+- renderer-neutral owned page buffers and affine pixel↔PDF transforms;
+- `OcrOptions` and opt-in `Off`/`Auto`/`Force` routing modes;
+- positioned OCR/layout results and per-page provenance types; and
+- a versioned PP-OCRv6 Small manifest with checksum-verified, locked, atomic
+  model-cache installation and explicit offline-directory overrides.
+
+```toml
+[dependencies]
+pdf-inspector = { version = "1", features = ["vision", "model-cache"] }
+```
+
+The OCR contracts preserve existing behavior by default: OCR is `Off`, learned
+layout is disabled, and model resolution is never reached. `ModelStore` itself
+does not access the network; a runtime integration can fetch a manifest's
+canonical URL only when allowed and pass the stream to `ModelStore::install`.
+Offline consumers set an explicit model directory and `ModelDownloadPolicy::Offline`.
+Renderer-only consumers do not enable `model-cache` and therefore do not compile
+its filesystem, locking, or hashing dependencies.
+
+```rust
+use pdf_inspector::vision::{
+    ModelDownloadPolicy, ModelStore, OcrMode, OcrOptions, PP_OCR_V6_SMALL,
+};
+
+let ocr = OcrOptions::new()
+    .mode(OcrMode::Auto)
+    .model_directory("/opt/firecrawl/models/pp-ocrv6-small")
+    .model_downloads(ModelDownloadPolicy::Offline);
+// Verifies exact sizes and SHA-256 digests before an engine opens the files.
+let models = ModelStore::from_options(&ocr)?.resolve(&PP_OCR_V6_SMALL)?;
+println!("using {} at {}", models.manifest_id(), models.revision());
+```
+
 ### Optional native page rendering
 
 The `render-pdfium` feature adds a native-only page renderer backed by
 [`firecrawl-pdfium`](https://crates.io/crates/firecrawl-pdfium). It is the
 rendering boundary for OCR pipelines; enabling it does not include an OCR
-model or change the existing extraction functions.
+model or change the existing extraction functions. It implies `vision`,
+and `PdfiumRenderer` implements the renderer-neutral `PageRenderer` trait.
 
 ```toml
 [dependencies]