Add a `solarize` filter

[swperb]

A nice self-contained first contribution: add a solarize filter — the photographic Sabattier effect, where channel values above a threshold are inverted.

Spec

• Name: solarize
• Usage: solarize[=threshold], threshold 0–255 (default 128)
• For each R/G/B channel c: if c >= threshold, set c = 255 - c; else leave it. Alpha untouched.

Where — mostly in src/filters.c

1. Write f_solarize. Copy f_invert (~line 249) as a template — same shape, just add the threshold check. Read the arg with argd(a, n, 0, 128.0) (see f_brightness, ~line 264). clampb() clamps to 0–255.
2. Register it in the FILTERS[] table (the "colour" block, ~line 578):

   {"solarize",    "solarize[=T]                    invert channels >= T (default 128)", f_solarize},

3. Document it: add solarize to the README Filters → Colour list (README.md). Per CONTRIBUTING.md ("Adding or changing a filter", step 5), a new filter must be listed there.

Done when

• make builds clean (-Wall -Wextra, no warnings)
• imgcli -filters lists solarize
• solarize appears in the README Colour filter list
• imgcli -y -i testsrc=128x128 -vf "solarize" out.png works; solarize=64 changes the result
• (bonus) a line in the check target of the Makefile exercising it

Small, visual, and a real feature. Difficulty: easy — happy to give pointers.