feat(core/org): add x25519-dalek dep + stub org module

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TJo44YM3UbBjro2fG6NrKy

Cargo.lock

@@ -2209,6 +2209,7 @@ dependencies = [
  "thiserror 2.0.18",
  "unicode-normalization",
  "url",
+ "x25519-dalek",
  "zeroize",
  "zstd",
  "zxcvbn",
@@ -3709,6 +3710,18 @@ version = "0.13.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ea6fc2961e4ef194dcbfe56bb845534d0dc8098940c7e5c012a258bfec6701bd"
 
+[[package]]
+name = "x25519-dalek"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7e468321c81fb07fa7f4c636c3972b9100f0346e5b6a9f2bd0603a52f7ed277"
+dependencies = [
+ "curve25519-dalek",
+ "rand_core",
+ "serde",
+ "zeroize",
+]
+
 [[package]]
 name = "yoke"
 version = "0.8.2"