fix: broken "Build and Deploy" badge link in README

[artcava]

Problem

The "Build and Deploy" status badge at the top of README.md points to a workflow file that no longer exists:

.github/workflows/master_xposterfunction.yml

This causes the badge to render as broken/unknown instead of showing the actual CI status.

Root Cause

The workflow was renamed from master_xposterfunction.yml to ci.yml but the badge reference in the README was never updated.

Fix

Update the badge URL in README.md from:

[![Build and Deploy](https://github.com/artcava/XPoster/actions/workflows/master_xposterfunction.yml/badge.svg)](https://github.com/artcava/XPoster/actions/workflows/master_xposterfunction.yml)

to:

[![Build and Deploy](https://github.com/artcava/XPoster/actions/workflows/ci.yml/badge.svg)](https://github.com/artcava/XPoster/actions/workflows/ci.yml)

Acceptance Criteria

• Badge in README.md points to ci.yml
• Badge renders correctly on the repository homepage

[github-actions]

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