Parallelize cucumber tests using GitHub Actions matrix

[afarber]

Issue

Fixes #7307

Add a new cucumber-tests job that runs cucumber.js tests in parallel using GitHub Actions matrix strategy.

The CI matrix creates 18 parallel jobs:

• 3 build configs (clang-18-release, gcc-14-release, conan-linux-release)
• 2 algorithms (CH, MLD)
• 3 load methods (datastore, mmap, directly)

Each combination runs as a separate job, so failures are easy to identify in the GitHub Actions UI.

The build configs were chosen to cover different compilers without running cucumber.js for every build variant (which would result in 108 test jobs).

Build artifacts are uploaded after the build step and downloaded by the cucumber test jobs.

Tasklist

• CHANGELOG.md entry (How to write a changelog entry)
• update relevant Wiki pages
• add tests (see testing documentation)
• review
• adjust for comments

Requirements / Relations

Link any requirements here. Other pull requests this PR is based on?