Feat: improve CLI with help menu and multi-file support

Extracts all argument-parsing and help logic into cli.rs:
- -h / --help: colored usage and options summary
- -V / --version: print version from Cargo.toml
- Unknown flags produce a styled error with a hint
- At least one file required; missing-file error is styled
- Multiple files accepted; exit code 1 if any file has parse errors

fluxc/src/cli.rs

@@ -0,0 +1,87 @@
+use std::process;
+
+use yansi::Paint as _;
+
+// ── Help ───────────────────────────────────────────────────────────────────────
+
+pub fn print_help() {
+    println!(
+        "{} {} — the Flux language compiler\n",
+        "fluxc".bold(),
+        env!("CARGO_PKG_VERSION").dim(),
+    );
+    println!("{}", "USAGE:".bold().yellow());
+    println!("  fluxc [OPTIONS] <file> [<file> ...]\n");
+    println!("{}", "OPTIONS:".bold().yellow());
+    println!(
+        "  {}, {}    Print this help message",
+        "-h".bold(),
+        "--help".bold(),
+    );
+    println!(
+        "  {}, {}  Print version information",
+        "-V".bold(),
+        "--version".bold(),
+    );
+    println!();
+    println!("{}", "ARGS:".bold().yellow());
+    println!(
+        "  {}    One or more Flux source files to compile",
+        "<file>".bold(),
+    );
+}
+
+pub fn print_version() {
+    println!("fluxc {}", env!("CARGO_PKG_VERSION"));
+}
+
+// ── Error helpers ─────────────────────────────────────────────────────────────
+
+pub fn fatal(msg: &str) -> ! {
+    eprintln!("{}: {}", "error".bold().red(), msg.bold());
+    eprintln!("  {} fluxc --help", "hint:".bold().cyan());
+    process::exit(1);
+}
+
+pub fn io_error(path: &str, err: std::io::Error) -> ! {
+    eprintln!(
+        "{}: cannot read {}: {}",
+        "error".bold().red(),
+        path.bold(),
+        err,
+    );
+    process::exit(1);
+}
+
+// ── Argument parsing ──────────────────────────────────────────────────────────
+
+pub struct Opts {
+    pub files: Vec<String>,
+}
+
+pub fn parse_args() -> Opts {
+    let mut files = Vec::new();
+
+    for arg in std::env::args().skip(1) {
+        match arg.as_str() {
+            "-h" | "--help" => {
+                print_help();
+                process::exit(0);
+            }
+            "-V" | "--version" => {
+                print_version();
+                process::exit(0);
+            }
+            flag if flag.starts_with('-') => {
+                fatal(&format!("unknown option `{flag}`"));
+            }
+            path => files.push(path.to_owned()),
+        }
+    }
+
+    if files.is_empty() {
+        fatal("no input files — at least one source file is required");
+    }
+
+    Opts { files }
+}

fluxc/src/main.rs

@@ -1,26 +1,33 @@
-use std::{env::args, fs};
+use std::{fs, process};
 
 use crate::parser::Parser;
 
 pub mod ast;
+pub mod cli;
 pub mod diagnostics;
 pub mod lexer;
 pub mod parser;
 pub mod token;
 
 fn main() {
-    let path = args().nth(1).expect("usage: fluxc <file>");
-    let content = fs::read_to_string(&path).unwrap_or_else(|e| {
-        eprintln!("error: {e}");
-        std::process::exit(1)
-    });
+    let opts = cli::parse_args();
+    let mut had_errors = false;
+
+    for path in &opts.files {
+        let content = fs::read_to_string(path).unwrap_or_else(|e| cli::io_error(path, e));
 
         let mut parser = Parser::new(&content);
         let program = parser.parse_program();
 
         for diag in &parser.errors {
-        eprint!("{}", diag.render(&content, &path));
+            eprint!("{}", diag.render(&content, path));
+            had_errors = true;
         }
 
         println!("{program:#?}");
     }
+
+    if had_errors {
+        process::exit(1);
+    }
+}