Implement 2025 day 5 in C

2025/day05/README.md

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+# Day 05: C
+
+C is weird. It's a very simple language but it's made complicated by the fact that you have to do
+everything yourself. Relatively straightforward solution.
+
+```console
+$ make
+cc -Wall -Wextra -O2 -g    solve.c   -o solve
+$ ./solve sample.txt
+Part1: 3
+Part2: 14
+```