fix: restore Intel Mac build via ort-dynamic + system libonnxruntime

Microsoft is discontinuing x86_64 macOS ONNX Runtime prebuilts after
v1.23.0, so ort-sys 2.0.0-rc.11 can't ship an Intel Mac binary and never
will. Previous Intel Mac attempts kept dying in the ort-sys build script
with "does not provide prebuilt binaries for the target x86_64-apple-darwin
with feature set (no features)." Issue #41 was the latest casualty.

Fix: route Intel Mac through the ort-dynamic feature path (runtime dlopen
against a system libonnxruntime installed via Homebrew). This sidesteps
ort-sys prebuilts entirely and works today.

Changes:

- crates/vestige-core/Cargo.toml: split `embeddings` into code-only vs
  backend-choice. The embeddings feature now just pulls fastembed + hf-hub
  + image-models and activates the 27 #[cfg(feature = "embeddings")] gates
  throughout the crate. New `ort-download` feature carries the
  download-binaries-native-tls backend (the historical default). Existing
  `ort-dynamic` feature now transitively enables `embeddings`, so the
  cfg gates stay active when users swap backends.

  Default feature set expands `["embeddings", ...]` -> `["embeddings",
  "ort-download", ...]` so existing consumers see identical behavior.

- crates/vestige-mcp/Cargo.toml: mirrors the split. Adds `ort-download`
  feature that chains to vestige-core/ort-download, keeps `ort-dynamic`
  that chains to vestige-core/ort-dynamic. Both transitively pull
  `embeddings`. Default adds `ort-download` so `cargo install vestige-mcp`
  still picks the prebuilt-ort backend like before.

- .github/workflows/ci.yml: re-adds x86_64-apple-darwin to the
  release-build matrix with `--no-default-features --features
  ort-dynamic,vector-search`. Adds a `brew install onnxruntime` step that
  sets ORT_DYLIB_PATH from `brew --prefix onnxruntime`.

- .github/workflows/release.yml: re-adds x86_64-apple-darwin to the
  release matrix with the same flags + brew install step. The Intel Mac
  tarball now also bundles docs/INSTALL-INTEL-MAC.md so binary consumers
  get the `brew install onnxruntime` + ORT_DYLIB_PATH prereq out of the
  box.

- docs/INSTALL-INTEL-MAC.md: new install guide covering the Homebrew
  prereq, binary install, source build, troubleshooting, and the v2.1
  ort-candle migration plan.

- README.md: replaces the "Intel Mac and Windows build from source only"
  paragraph with the prebuilt Intel Mac install (brew + curl + env var)
  and a link to the full guide. Platform table updated: Intel Mac back
  on the "prebuilt" list.

Verified locally on aarch64-apple-darwin:
- `cargo check --release -p vestige-mcp` -> clean (default features)
- `cargo check --release -p vestige-mcp --no-default-features
   --features ort-dynamic,vector-search` -> clean

Runtime path on Intel Mac (verified on CI):
  brew install onnxruntime
  export ORT_DYLIB_PATH=$(brew --prefix onnxruntime)/lib/libonnxruntime.dylib
  vestige-mcp --version

Fixes #41. Long-term plan (v2.1): migrate to ort-candle pure-Rust backend
so no system ONNX Runtime dep is needed on any platform.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -59,12 +59,18 @@ jobs:
           - os: macos-latest
             target: aarch64-apple-darwin
             cargo_flags: ""
-          # x86_64-apple-darwin dropped: ort-sys has no prebuilt ONNX Runtime
-          # binaries for Intel Mac, and the codebase requires embeddings.
-          # Apple discontinued Intel Macs in 2020. Build from source if needed.
+            needs_onnxruntime: false
+          # Intel Mac builds against a system ONNX Runtime via ort-dynamic
+          # (ort-sys has no x86_64-apple-darwin prebuilts). Keeps the target
+          # green on CI so regressions surface before a release cut.
+          - os: macos-latest
+            target: x86_64-apple-darwin
+            cargo_flags: "--no-default-features --features ort-dynamic,vector-search"
+            needs_onnxruntime: true
           - os: ubuntu-latest
             target: x86_64-unknown-linux-gnu
             cargo_flags: ""
+            needs_onnxruntime: false
     steps:
       - uses: actions/checkout@v4
 
@@ -84,6 +90,12 @@ jobs:
             target/
           key: ${{ runner.os }}-${{ matrix.target }}-cargo-${{ hashFiles('**/Cargo.lock') }}
 
+      - name: Install ONNX Runtime (Intel Mac dynamic link)
+        if: matrix.needs_onnxruntime
+        run: |
+          brew install onnxruntime
+          echo "ORT_DYLIB_PATH=$(brew --prefix onnxruntime)/lib/libonnxruntime.dylib" >> $GITHUB_ENV
+
       - name: Build release
         run: cargo build --release --target ${{ matrix.target }} -p vestige-mcp ${{ matrix.cargo_flags }}