ci: enable Rust build artifact caching in Python tests (#143)

## Summary
Optimize CI workflows to dramatically reduce build times through
improved caching and build configuration.

## Changes

### 1. Shared Rust Dependency Cache
- Added `shared-key: "lance-graph-deps"` to all workflows
- Cache is shared across Python, Rust, Build, and Style workflows
- Include both `crates/lance-graph` and `crates/lance-graph-python`
workspaces
- Prevents duplicate compilation across different workflows

### 2. Fix Double Rust Build in Python Tests
- Removed `uv pip install -e .[tests]` which was triggering Rust
compilation
- Now only `maturin develop` builds the extension once
- **Result: 24 min → 6 min (74% faster!)**

### 3. Remove Nightly from Rust Test Matrix
- Only test on stable toolchain (matches upstream lance project)
- Prevents nightly breakage from blocking PRs
- Stable is the primary deployment target

### 4. Explicit CARGO_INCREMENTAL=0
- Set explicitly for CI (rust-cache overrides to 0 anyway)
- Makes intention clear: incremental artifacts not useful in CI

## Performance Impact

**Before:**
- Python Tests: ~24 minutes
- Rust compiled twice (18min + 5min)
- No cache sharing across workflows

**After:**
- Python Tests: **6 minutes (74% faster)** ✅
- Rust compiled once (5min)
- Shared cache across all workflows

## Test Results

All CI checks passing:
- ✅ Python Tests (test 3.11) - 6 minutes
- ✅ Rust Tests (stable)
- ✅ Rust Coverage
- ✅ Build (stable)
- ✅ All Linters (clippy, format, Python lint, spell check)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: beinan

.github/workflows/build.yml

@@ -36,8 +36,10 @@ jobs:
           rustup default ${{ matrix.toolchain }}
       - uses: Swatinem/rust-cache@v2
         with:
-          workspaces: crates/lance-graph
-          cache-targets: false
+          shared-key: "lance-graph-deps"
+          workspaces: |
+            crates/lance-graph
+            crates/lance-graph-python
       - name: Install dependencies
         run: |
           sudo apt update

.github/workflows/python-test.yml

@@ -18,6 +18,7 @@ concurrency:
 env:
   CARGO_TERM_COLOR: always
   RUSTFLAGS: "-C debuginfo=1"
+  CARGO_INCREMENTAL: "0"
 
 jobs:
   test:
@@ -36,8 +37,10 @@ jobs:
         uses: actions-rust-lang/setup-rust-toolchain@v1
       - uses: Swatinem/rust-cache@v2
         with:
-          workspaces: python
-          cache-targets: false
+          shared-key: "lance-graph-deps"
+          workspaces: |
+            crates/lance-graph
+            crates/lance-graph-python
       - name: Install dependencies
         run: |
           sudo apt update
@@ -50,7 +53,8 @@ jobs:
           uv venv
           source .venv/bin/activate
           uv pip install maturin[patchelf]
-          uv pip install -e .[tests]
+          # Install test dependencies only (skip editable install to avoid double-building Rust)
+          uv pip install pytest pyarrow pandas ruff
       - name: Build Python extension
         working-directory: python
         run: |

.github/workflows/rust-test.yml

@@ -28,7 +28,6 @@ jobs:
       matrix:
         toolchain:
           - stable
-          - nightly
     steps:
       - uses: actions/checkout@v4
       - name: Setup rust toolchain
@@ -37,8 +36,10 @@ jobs:
           rustup default ${{ matrix.toolchain }}
       - uses: Swatinem/rust-cache@v2
         with:
-          workspaces: crates/lance-graph
-          cache-targets: false
+          shared-key: "lance-graph-deps"
+          workspaces: |
+            crates/lance-graph
+            crates/lance-graph-python
       - name: Install dependencies
         run: |
           sudo apt update
@@ -61,8 +62,10 @@ jobs:
           rustup default stable
       - uses: Swatinem/rust-cache@v2
         with:
-          workspaces: crates/lance-graph
-          cache-targets: false
+          shared-key: "lance-graph-deps"
+          workspaces: |
+            crates/lance-graph
+            crates/lance-graph-python
       - name: Install dependencies
         run: |
           sudo apt update

.github/workflows/style.yml

@@ -37,7 +37,10 @@ jobs:
       - uses: actions/checkout@v4
       - uses: Swatinem/rust-cache@v2
         with:
-          workspaces: crates/lance-graph
+          shared-key: "lance-graph-deps"
+          workspaces: |
+            crates/lance-graph
+            crates/lance-graph-python
       - name: Install dependencies
         run: |
           sudo apt update