Add model_manifest.json for the radgraphit package

Records package/schema/backend compatibility, the medBIT encoder, and
SHA-256 checksums for config.json, vocab.json and best.pt. Enables
integrity-verified loading via RadGraphIT.from_pretrained().

model/model_manifest.json

@@ -0,0 +1,24 @@
+{
+  "manifest_version": 1,
+  "package": "radgraphit",
+  "min_package_version": "0.1.0",
+  "schema": "radgraph-xl",
+  "backend": "dygie_v2",
+  "encoder": {
+    "model_name": "IVN-RIN/medBIT-r3-plus",
+    "max_length": 512
+  },
+  "revision": "v1",
+  "weights_file": "best.pt",
+  "files": {
+    "config.json": {
+      "sha256": "378f0c083f71662e204c8876b805ba8298f7a056db4b6cc64b95079a0d476d23"
+    },
+    "vocab.json": {
+      "sha256": "e32c6ab26dad1625dfc3f5fa2bcf91d466ba7cf231da0ab78846eae84ad20f76"
+    },
+    "best.pt": {
+      "sha256": "987fc75edf3ac78d1a0f05b8260f832c16ccd0cb6d6612432b2c0dbdf00a757a"
+    }
+  }
+}