build: raise MSRV to Rust 1.94

[XiaoHongbo-Hope]

Purpose

Fix the declared MSRV for Apple Silicon before the 0.4.0 release.

paimon-vindex-core 0.4.0 uses AArch64 NEON FP16 intrinsics that became stable in Rust 1.94.0. The previous x86_64-only MSRV job did not compile that target-specific path, so Rust 1.91 through 1.93 failed on aarch64-apple-darwin even though the workspace declared Rust 1.91.0.

No linked issue.

Brief change log

• Raise the workspace rust-version from 1.91.0 to 1.94.0.
• Run the MSRV check on Linux x86_64 and macOS AArch64.
• Assert the actual Rust host on each MSRV runner.
• Move the Darwin ARM64 release build from the deprecated macos-14 image to macos-15.

Tests

• cargo +1.94.0 check --locked --workspace --all-targets --all-features on Apple Silicon
• cargo clippy --locked --all-targets --workspace --features fulltext,vortex -- -D warnings on Apple Silicon with Rust 1.94.1
• cargo +1.94.0 fmt --all -- --check
• Cargo metadata verifies all workspace packages declare Rust 1.94.0
• GitHub Actions workflow YAML parsing
• git diff --check

API and Format

No API or storage-format changes. This changes the minimum supported Rust version.

Documentation

No documentation files changed. The 0.4.0 release notes should call out the MSRV increase from Rust 1.91 to 1.94.

[JingsongLi]

+1