Fix: reuse Header instance in Cron formatter

[zigzagdev]

Description

This PR ensures Cron::header() returns the same Header instance instead of creating a new one on every call. It also adds regression tests to prevent state loss when header() is called multiple times before formatting.

Motivation

header() previously reinitialised the Header each time, which could silently drop previously configured header state (e.g. PATH, MAILTO) when chaining multiple header calls. Returning a single instance makes the behaviour predictable and preserves configuration until format() is called.

What changed

• Updated Cron::header() to lazily initialise the Header once and reuse it for subsequent calls.
• Added tests to confirm:
  • header() returns the same instance (assertSame)
    Header state is preserved across multiple calls and appears in the formatted output.

Test plan & results

composer install

./vendor/bin/phpunit