Fix PHP minimum version readiness check

[fetachino]

Closes #38.

Aligns check_readiness() with Composer's declared PHP 7.4 minimum and updates the runtime error message accordingly.

The readiness function now accepts an optional version argument so the compatibility boundary can be tested without depending on the PHP version running the suite. Unit coverage verifies that 7.4.0 is accepted and 7.3.99 produces the expected E_USER_ERROR.

Validation:

• git diff --check — passed
• PHP/Composer tests were not run locally because neither executable is installed on this Windows host; the repository CI matrix will exercise PHP 7.4 through 8.5.