Add a tiles effect

The two-dimensional counterpart of scanlines: rectangular tiles cut at
random positions from the source images, in three arrangements. A grid
draws its column widths and row heights once so the cells stay aligned
even when jittered; brick repacks every row on its own so the vertical
seams do not meet; scatter fills the canvas from one of the images and
drops loose tiles over it.

The plan is drawn serially from the rng as scanlines does, but scattered
tiles overlap, so they are painted in plan order on one thread: letting
threads race for the same pixels would leave the output depending on
their scheduling. A source smaller than a tile is repeated to fill the
cell, sideways and then downwards, which keeps grid and brick gapless.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -14,8 +14,8 @@ Everything is warning-clean under `-Wall -Wextra -Wpedantic`; keep it that way.
 
 The test suite is a single hand-rolled binary (no framework), so there is no
 per-test filter: run `./build/tests/effector_tests` directly to see the section
-headings (`rng`, `registry`, `scanlines`, `blur`, `chain`, `project`, `image
-io`, `batch loading`) and the `ok`/`FAIL` lines. To iterate on one area, comment
+headings (`rng`, `registry`, `scanlines`, `tiles`, `blur`, `chain`, `project`,
+`image io`, `batch loading`) and the `ok`/`FAIL` lines. To iterate on one area, comment
 out the other blocks in `tests/core_tests.cpp` or add a new
 `std::cout << "section\n"` block — checks are plain
 `check(condition, "description")` calls and `main()` returns the failure count.