Add an Arabic README as an entry point

[eaitbrahim]

A large share of the intended audience reads Arabic. An Arabic entry point is a strong signal that the project means to serve them rather than merely permit them.

Acceptance

• README.ar.md covering the first-screen content: what keel is, the honest measured result, the "not a fatwa engine" boundary, and the quickstart
• Language switcher links at the top of both READMEs
• Terminology reviewed by a native speaker familiar with Islamic finance vocabulary — riba, qabd, 'ayn, dayn, gharar have precise established renderings and a machine translation will get them subtly wrong
• Scope stated: the full docs remain English; this is the entry point, not a promise to translate everything

Note

Do not attempt this before the English README is rewritten (Phase 7) — translating a document that is about to change wastes the reviewer's goodwill.