Add C language support (compiled_lang)

[darkn3rd]

Add C as a new language in lessons/compiled_lang/, alongside the existing C++/C#/Go/Java/Rust.

Based on cpp/ as the reference (not go/ - C shares C++'s two-stage compile-then-link build model via make, unlike Go/Rust's single-step toolchain compilers), since C and C++ also share the same underlying compiler toolchain:

• New lessons/compiled_lang/c/ directory: Makefile(.win) (adapted from cpp/Makefile - CC := gcc, CFLAGS := -std=c17 -O2 -Wall, -static-libgcc on Windows for the same DLL-mismatch reason cpp/Makefile already documents, no -static-libstdc++ since C doesn't link libstdc++), Rakefile (imports shared testbox.rake), psakefile.ps1, src/ with one .c file per lesson (~60 files mirroring cpp/src's set, minus anything that's specifically a C++ language feature rather than a general lesson topic), bin//target/ output dirs
• No new entries needed in Brewfile/choco.config/cygwin_pkgs.txt/msys2_pkgs.txt - confirm the existing C++ toolchain packages (gcc-core/gcc-g++, mingw-w64-x86_64-toolchain, visualstudio2022buildtools w/ VCTools, Homebrew's make + Xcode CLI tools) already provide a working C compiler on every platform, since gcc/clang ship C support alongside C++
• Add a C entry to the Compiled Languages area in scriptbox/scripts/verify_commands.rb's AREAS
• lessons/compiled_lang/README.md update

This is a full new-language port (~60 lesson files), so likely worth breaking into smaller PRs rather than one large one.