Replace Shields README badges with Badgen

[acgetchell]

README currently uses img.shields.io for the Crates.io, downloads, and license badges. Those badges are no longer reliably generating valid SVGs.

Replace them with Badgen endpoints matching the pattern already verified in acgetchell/delaunay:

• Crates.io: https://badgen.net/crates/v/la-stack
• Downloads: https://badgen.net/crates/d/la-stack
• License: https://badgen.net/github/license/acgetchell/la-stack

Keep the existing badge target links to crates.io and LICENSE.