Remove macOS x86_64 builds - Intel runners retired

GitHub has retired macOS Intel runners (macos-12/13).
Intel Mac users can install from sdist and get pure Python fallback.

Wheels now built for:
- Linux x86_64 (with Rust backend)
- macOS ARM64 (with Rust backend)
- Source distribution (pure Python fallback for all other platforms)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Merge pull request #55 from igerber/claude/v1.4.0-performance-improve…

…ments

feat: Major performance improvements for v1.4.0

Merge pull request #52 from igerber/claude/prepare-v1.3-release-Np8dK

Prepare codebase for v1.3 release

Merge pull request #49 from igerber/claude/prepare-v1.3.0-release-XmIcZ

Merge pull request #47 from igerber/claude/review-pr-46-YRDFo

Merge pull request #45 from igerber/claude/brainstorm-next-project-YjI2n

Merge pull request #43 from igerber/claude/prep-v1.1.0-release-SHH8a

Merge pull request #34 from igerber/claude/prep-1-0-release-uDE3T

Prepare for 1.0.0 release

Merge pull request #28 from igerber/claude/prepare-0.6.0-release-V6fGP

Prepare 0.6.0 release

Merge pull request #19 from igerber/claude/validate-readme-examples-u…

…Ryan