Feat: add -S flag to emit LLVM IR

`-S` stops the pipeline after IR emission and writes the `.ll` file
directly to the output path (default `<stem>.ll`). It implies `-c`
(no main required). Combined with `-o`, the IR goes to the specified
path.

Pipeline summary:
  (none)  →  emit → opt → llc → cc  →  executable
  -c      →  emit → opt → llc       →  <stem>.o
  -S      →  emit                   →  <stem>.ll

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

fluxc/src/cli.rs

@@ -27,6 +27,10 @@ pub fn print_help() {
         "  {}              Compile to object file (no `main` required, no linking)",
         "-c".bold(),
     );
+    println!(
+        "  {}              Emit LLVM IR and stop (implies `-c`)",
+        "-S".bold(),
+    );
     println!(
         "  {} {}      Write output to <file>",
         "-o".bold(),
@@ -68,6 +72,8 @@ pub struct Opts {
     pub files: Vec<String>,
     /// `-c`: compile to object file without requiring a `main` entry point.
     pub no_main: bool,
+    /// `-S`: emit LLVM IR text and stop (implies `-c`).
+    pub emit_ir: bool,
     /// `-o <file>`: write final output to this path.
     pub output: Option<String>,
 }
@@ -75,6 +81,7 @@ pub struct Opts {
 pub fn parse_args() -> Opts {
     let mut files = Vec::new();
     let mut no_main = false;
+    let mut emit_ir = false;
     let mut output: Option<String> = None;
     let mut args = std::env::args().skip(1).peekable();
 
@@ -89,6 +96,7 @@ pub fn parse_args() -> Opts {
                 process::exit(0);
             }
             "-c" => no_main = true,
+            "-S" => { emit_ir = true; no_main = true; }
             "-o" => match args.next() {
                 Some(path) => output = Some(path),
                 None => fatal("option `-o` requires an argument"),
@@ -104,9 +112,5 @@ pub fn parse_args() -> Opts {
         fatal("no input files — at least one source file is required");
     }
 
-    Opts {
-        files,
-        no_main,
-        output,
-    }
+    Opts { files, no_main, emit_ir, output }
 }