refactor: optimize prepQuotedPrintable() with hash lookup and int-length tracking

[gr8man]

Description

Optimize prepQuotedPrintable() in system/Email/Email.php by replacing three expensive patterns in the hot loop:

• in_array($ascii, $asciiSafeChars) → isset($asciiSafeChars[$ascii]) using array_fill_keys()
• static::strlen($temp) + static::strlen($char) → integer $tempLen variable, incremented/decremented
• $escape . strtoupper(sprintf('%02s', dechex($ascii))) + static::strlen($char) → sprintf('=%02X', $ascii) with preset $charLen = 3

Added unit tests covering safe chars, unsafe chars, trailing space/tab, =3D, soft line breaks at 76 chars, {unwrap} removal, null-byte stripping, both CRLF paths, and mixed content.

[michalsn]

Please restore the comments that remain relevant after the refactor, especially those explaining the {unwrap} removal, the custom-LF compatibility path, encoding of =, and the line-wrapping rules. These capture non-obvious behavior and will make our lives easier in the future.

[michalsn]

Thank you!

[paulbalandan]

Thank you, @gr8man