ci: optimize build times with shared caching and streamlined workflows

[beinan]

Summary

Apply learnings from lance-graph#143 to dramatically improve CI performance. That PR achieved a 74% speedup (24min → 6min) in Python tests through better caching and eliminating duplicate builds.

Changes

1. Shared Rust Cache Across All Workflows ⭐

• Switched from manual cache@v4 to Swatinem/rust-cache@v2
• Added shared-key: "lance-context-deps" to all workflows
• Impact: Prevents duplicate compilation of Rust dependencies across Python, Rust, and Style workflows
• Cache is now shared across all jobs instead of being isolated

2. Remove Nightly from Test Matrix

• Only test on stable toolchain (matches upstream lance project)
• Impact: ~50% reduction in Rust test time, prevents nightly breakage from blocking PRs
• Stable is the primary deployment target

3. Standardize CARGO_INCREMENTAL=0

• Added to python-test.yml (was missing)
• Added to style.yml (was missing)
• Already present in rust-test.yml
• Impact: Consistent CI environment, no wasted incremental compilation artifacts

4. Optimized Cache Configuration

• Removed CARGO_TARGET_DIR override (rust-cache handles this better)
• Standardized workspaces across all workflows to include both crates/lance-context-core and python
• Impact: Better cache hit rates and management

Expected Performance Impact

Based on the improvements seen in lance-graph:

• ✅ Reduced duplicate Rust compilations via shared cache key
• ✅ Faster CI runs with streamlined test matrix
• ✅ Better cache efficiency with rust-cache action
• ✅ Consistent build environment across all workflows

Test Plan

• All workflow files updated consistently
• Verify CI passes with new caching strategy
• Monitor CI times to confirm performance improvement

🤖 Generated with Claude Code