feat: SIMD compilation mode - CPU portable vs native mode selection

[nyurik]

This pull request introduces a new SIMD mode configuration system for the FastPFOR crate, allowing users to select between portable, native, and runtime SIMD instruction sets for the C++ backend. The changes improve build flexibility, CI reliability, and documentation clarity, while updating the crate version and standardizing feature usage in development scripts.

SIMD Mode Configuration and Build System

• Added support for selecting SIMD mode (portable, native, or runtime) via Cargo features and the FASTPFOR_SIMD_MODE environment variable; the build script now warns if multiple SIMD features are enabled and passes the selected mode to CMake. (build.rs, Cargo.toml) [1] [2]
• Updated GitHub Actions CI workflow to test all combinations of OS and SIMD mode, and to cache builds separately for each SIMD mode. (.github/workflows/ci.yml)

Documentation Improvements

• Expanded README.md with a clear explanation of SIMD modes, their use cases, and configuration recommendations for end users.

Development Scripts and Testing

• Standardized the use of the _all_compatible feature group in the justfile for building, testing, linting, and coverage, and added recipes to test all SIMD modes. [1] [2] [3] [4]
• Improved installation logic in the justfile for development tools, with clearer output and better CI compatibility.

Version Update

• Bumped the crate version from 0.7.0 to 0.8.0 to reflect these breaking and feature enhancements. (Cargo.toml)

[codecov]

Codecov Report

❌ Patch coverage is 55.55556% with 16 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
build.rs	55.55%	16 Missing ⚠️

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[Copilot]

Pull request overview

Introduces a configurable SIMD compilation mode for the FastPFOR C++ backend (portable vs native vs runtime), improving build/CI flexibility and clarifying usage for consumers.

Changes:

• Adds SIMD mode selection via Cargo features and FASTPFOR_SIMD_MODE, wiring the selection through build.rs into CMake.
• Updates dev workflows (justfile) and CI matrix to exercise multiple SIMD modes.
• Updates docs and bumps crate version to 0.8.0.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
justfile	Standardizes dev commands on _all_compatible and adds SIMD-mode test recipes.
build.rs	Detects SIMD mode via features/env var and forwards it to CMake with warnings.
README.md	Documents SIMD modes and configuration knobs for end users.
Cargo.toml	Adds SIMD-related features and bumps crate version.
.github/workflows/ci.yml	Expands CI matrix to test multiple SIMD modes and separates caches per mode.

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