ci: run mutation tests with Infection

[roxblnfk]

Q	A
Bugfix?	❌
Breaks BC?	❌
New feature?	❌
Issues	—

Adds mutation testing with Infection on top of the Testo suite.

• Mutation Tests workflow runs on PRs and pushes to 1.x (PHP 8.4, Xdebug, zend.assertions=1) with --min-msi=90 --min-covered-msi=90; pushes to 1.x send the report to the Stryker dashboard, and the README gets its badge.
• Local run: 38 mutants, MSI 92%, covered MSI 92%. A new data set checks that a float TTL is rounded to the nearest microsecond (kills the floor/ceil mutants). The 3 escaped mutants are equivalent: round/floor/ceil and the (int) cast on the already integral DateInterval product.

Review notes

• Infection is installed with composer require in the workflow, not from the phar: the Testo adapter bundled in the 0.35.6 phar writes a bootstrap calling the unscoped \Infection\StreamWrapper\IncludeInterceptor, while the phar only contains the scoped class, so every mutant ends as an error.

• It is not in require-dev because Infection ≥ 0.33 needs PHP 8.3+, and the package supports 8.2.

• How was this tested:

  • Infection and Testo run locally (PHP 8.4, Windows)

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