Feat: add -c flag to skip entry-point check

`fluxc -c <file>` compiles a source file without requiring a `main`
function, matching the convention of C compilers. Pass 4 (entry-point
validation) is skipped when `no_main` is set.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fluxc/src/cli.rs

@@ -23,6 +23,10 @@ pub fn print_help() {
         "-V".bold(),
         "--version".bold(),
     );
+    println!(
+        "  {}              Compile without requiring a `main` entry point",
+        "-c".bold(),
+    );
     println!();
     println!("{}", "ARGS:".bold().yellow());
     println!(
@@ -57,10 +61,13 @@ pub fn io_error(path: &str, err: std::io::Error) -> ! {
 
 pub struct Opts {
     pub files: Vec<String>,
+    /// `-c`: compile without requiring a `main` entry point.
+    pub no_main: bool,
 }
 
 pub fn parse_args() -> Opts {
     let mut files = Vec::new();
+    let mut no_main = false;
 
     for arg in std::env::args().skip(1) {
         match arg.as_str() {
@@ -72,6 +79,7 @@ pub fn parse_args() -> Opts {
                 print_version();
                 process::exit(0);
             }
+            "-c" => no_main = true,
             flag if flag.starts_with('-') => {
                 fatal(&format!("unknown option `{flag}`"));
             }
@@ -83,5 +91,5 @@ pub fn parse_args() -> Opts {
         fatal("no input files — at least one source file is required");
     }
 
-    Opts { files }
+    Opts { files, no_main }
 }