test(C1.1): cSourceCacheKey — content key for compiled #source units

Source bytes, declared-header CONTENT (header edits invalidate),
defines/flags/include dirs in order, LLVM version, and target
triple/sysroot all participate; section tags keep equal strings in
different roles distinct. Pure function + variance property tests;
nothing consumes it yet.

src/root.zig

@@ -16,6 +16,7 @@ pub const imports = @import("imports.zig");
 pub const imports_tests = @import("imports.test.zig");
 pub const core = @import("core.zig");
 pub const c_import = @import("c_import.zig");
+pub const c_import_tests = @import("c_import.test.zig");
 pub const ir = @import("ir/ir.zig");
 
 pub const lsp = struct {