Add coverage collection and reporting for project packages

[coderabbitai]

Overview

Currently, the .github/workflows/.lint.yml reusable workflow enables xdebug via shivammathur/setup-php (line 40), but no coverage data is actually collected or reported. This means there is unnecessary overhead on every lint shard without any benefit.

Goal

Implement proper code coverage generation, collection, and display for all packages in the monorepo.

Acceptance Criteria

• Add a dedicated coverage step/job (or extend the existing test workflow) to collect coverage data per package.
• Configure xdebug (or an appropriate coverage driver) only where coverage is actually gathered.
• Publish/display coverage stats (e.g., via a coverage badge, Codecov, Coveralls, or a similar service).
• Remove coverage: xdebug from the lint workflow (.github/workflows/.lint.yml) once coverage is handled in a dedicated workflow (or set it to none).

Context

Identified during code review on PR #47 (#47), see comment: #47 (comment).

Requested by @juliolmuller.