feat: add solarize filter and update documentation

[I-got-mine]

added the solarize filter and changed documentation per description. also cleaned up some if statements in rotate that were preventing make from completing. passes tests.

[swperb]

Merged — thank you, this is a great contribution! 🎉

I built it locally and ran the full suite to confirm: clean build with no warnings, 38/38 integration tests pass, solarize and solarize=64 both produce correct output, and rotate still expands the canvas as expected.

A few things I appreciated:

• The solarize implementation is clean and idiomatic — it follows the in-place point-filter convention, leaves the alpha channel untouched, and the optional [=T] threshold (default 128) is a nice touch.
• Registered correctly in the FILTERS[] table and documented in the README's Colour list — exactly the pattern we ask for in CONTRIBUTING.md.
• The brace cleanup in f_rotate is welcome too; one statement per line with explicit braces is the style we prefer.

Tiny non-blocking nits I'll tidy in a follow-up so I'm not bouncing the PR back to you: the help string says "invert channels >= T" while the code uses >, and there's a stray double-space. Nothing that affects behaviour.

Thanks again for adding a filter and leaving the surrounding code a little better than you found it. Hope to see more from you! 🙏