feat(build): add .core/build.yaml for core CLI integration

C++ build configuration for core CLI project detection and future
`core compile` command. Defines Conan dependencies, CMake settings,
cross-compilation targets, and packaging options.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.core/build.yaml

@@ -0,0 +1,83 @@
+# Lethean Blockchain Build Configuration
+# Used by: core compile
+
+version: 1
+
+project:
+  name: blockchain
+  type: cpp
+  description: "Lethean VPN Blockchain"
+  version: "6.0.1"
+
+cpp:
+  standard: 17
+  build_type: Release
+  static: false
+
+  conan:
+    version: "2.21.0"
+    requires:
+      - zlib/1.3.1
+      - boost/1.85.0
+      - openssl/3.2.0
+      - miniupnpc/2.2.5
+      - jwt-cpp/0.7.1
+      - oatpp/1.3.0.latest
+      - oatpp-swagger/1.3.0.latest
+    tool_requires:
+      - cmake/3.31.9
+    options:
+      boost/*:without_test: true
+    registry:
+      url: http://forge.snider.dev:4000/api/packages/host-uk/conan
+      remote: conan_build
+
+  cmake:
+    minimum_version: "3.16"
+    variables:
+      USE_CCACHE: "ON"
+
+  options:
+    testnet: false
+
+targets:
+  - os: linux
+    arch: x86_64
+    profile: gcc-linux-x86_64
+  - os: linux
+    arch: arm64
+    profile: gcc-linux-armv8
+  - os: darwin
+    arch: arm64
+    profile: apple-clang-armv8
+  - os: darwin
+    arch: x86_64
+    profile: apple-clang-x86_64
+  - os: windows
+    arch: x86_64
+    profile: msvc-194-x86_64
+
+package:
+  generators:
+    - TGZ
+    - ZIP
+    - NSIS
+    - DEB
+    - RPM
+  vendor: Lethean
+  contact: support@lt.hn
+  website: https://lt.hn
+
+docker:
+  dockerfile: utils/docker/lthn-chain/Dockerfile
+  image: lthn/chain
+  platforms:
+    - linux/amd64
+  tags:
+    - testnet
+    - "{{.Version}}"
+  build_args:
+    BUILD_THREADS: auto
+    BUILD_TESTNET: "1"
+    BUILD_STATIC: "0"
+    BUILD_TYPE: Release