Add Day 12 solution (ft. an unused DLX library)

Turns out two simple heuristics solve the whole problem in 4ms:
1. Does the area fit? No → invalid
2. Are there enough 3x3 spaces for all presents? Yes → valid

But hey, at least we learned how to use Dancing Links for
a problem that didn't need it. The dlx crate thanks us for
the download stats.

Cargo.toml

@@ -24,6 +24,7 @@ test_lib = []
 # Template dependencies
 chrono = { version = "0.4.38", optional = true }
 dhat = { version = "0.3.3", optional = true }
+dlx = "0.8.5"
 itertools = "0.14.0"
 pico-args = "0.5.0"
 rayon = "1.11.0"