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/release.yml

@@ -27,21 +27,27 @@ jobs:
             os: ubuntu-latest
             archive: tar.gz
             cargo_flags: ""
+            needs_onnxruntime: false
           - target: x86_64-pc-windows-msvc
             os: windows-latest
             archive: zip
             cargo_flags: ""
-          # Intel Mac (x86_64-apple-darwin) is explicitly unsupported: the
-          # upstream ort-sys 2.0.0-rc.11 pinned by fastembed 5.13.2 does not
-          # ship Intel Mac prebuilts, and the v2.0.5 + v2.0.6 release
-          # workflows both failed this job. Matches ci.yml which already
-          # dropped the target. README documents the build-from-source path
-          # for Intel Mac users. When ort-sys ships Intel Mac prebuilts
-          # again, restore the entry.
+            needs_onnxruntime: false
+          # Intel Mac uses the ort-dynamic feature to runtime-link against a
+          # system libonnxruntime (Homebrew), sidestepping the missing
+          # x86_64-apple-darwin prebuilts in ort-sys 2.0.0-rc.11. Binary
+          # consumers must `brew install onnxruntime` before running — see
+          # INSTALL-INTEL-MAC.md bundled in the tarball.
+          - target: x86_64-apple-darwin
+            os: macos-latest
+            archive: tar.gz
+            cargo_flags: "--no-default-features --features ort-dynamic,vector-search"
+            needs_onnxruntime: true
           - target: aarch64-apple-darwin
             os: macos-latest
             archive: tar.gz
             cargo_flags: ""
+            needs_onnxruntime: false
 
     steps:
       - name: Checkout
@@ -52,14 +58,25 @@ jobs:
         with:
           targets: ${{ matrix.target }}
 
+      - 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
         run: cargo build --package vestige-mcp --release --target ${{ matrix.target }} ${{ matrix.cargo_flags }}
 
       - name: Package (Unix)
         if: matrix.os != 'windows-latest'
         run: |
+          cp docs/INSTALL-INTEL-MAC.md target/${{ matrix.target }}/release/ 2>/dev/null || true
           cd target/${{ matrix.target }}/release
-          tar -czf ../../../vestige-mcp-${{ matrix.target }}.tar.gz vestige-mcp vestige vestige-restore
+          if [ "${{ matrix.target }}" = "x86_64-apple-darwin" ]; then
+            tar -czf ../../../vestige-mcp-${{ matrix.target }}.tar.gz vestige-mcp vestige vestige-restore INSTALL-INTEL-MAC.md
+          else
+            tar -czf ../../../vestige-mcp-${{ matrix.target }}.tar.gz vestige-mcp vestige vestige-restore
+          fi
 
       - name: Package (Windows)
         if: matrix.os == 'windows-latest'