Feat: add LLVM IR backend with opt/llc pipeline

Implements a full LLVM IR text emitter and three-step toolchain:
  1. Emit LLVM IR (.ll) via alloca-based codegen (mem2reg-friendly)
  2. `opt -O2` → optimised IR          (override with FLUXC_OPT)
  3. `llc -filetype=obj` → object file  (override with FLUXC_LLC)
  4. `cc` → link into executable        (override with FLUXC_CC)
     (step 4 skipped in -c mode)

Emitter supports all Flux types, operators, control flow (if/else,
while, loop, break, continue), structs, arrays, pointer operations,
function calls, string literals, and integer literal type inference
via UnboundInt → concrete-type coercion.

Also adds -o <file> CLI flag, exposes CheckResult from the checker
(sigma + phi tables reused by codegen), and updates main.rs to run
the full parse → check → codegen pipeline.

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

fluxc/src/cli.rs

@@ -24,9 +24,14 @@ pub fn print_help() {
         "--version".bold(),
     );
     println!(
-        "  {}              Compile without requiring a `main` entry point",
+        "  {}              Compile to object file (no `main` required, no linking)",
         "-c".bold(),
     );
+    println!(
+        "  {} {}      Write output to <file>",
+        "-o".bold(),
+        "<file>".bold(),
+    );
     println!();
     println!("{}", "ARGS:".bold().yellow());
     println!(
@@ -61,15 +66,19 @@ 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.
+    /// `-c`: compile to object file without requiring a `main` entry point.
     pub no_main: bool,
+    /// `-o <file>`: write final output to this path.
+    pub output: Option<String>,
 }
 
 pub fn parse_args() -> Opts {
     let mut files = Vec::new();
     let mut no_main = false;
+    let mut output: Option<String> = None;
+    let mut args = std::env::args().skip(1).peekable();
 
-    for arg in std::env::args().skip(1) {
+    while let Some(arg) = args.next() {
         match arg.as_str() {
             "-h" | "--help" => {
                 print_help();
@@ -80,6 +89,10 @@ pub fn parse_args() -> Opts {
                 process::exit(0);
             }
             "-c" => no_main = true,
+            "-o" => match args.next() {
+                Some(path) => output = Some(path),
+                None => fatal("option `-o` requires an argument"),
+            },
             flag if flag.starts_with('-') => {
                 fatal(&format!("unknown option `{flag}`"));
             }
@@ -91,5 +104,9 @@ pub fn parse_args() -> Opts {
         fatal("no input files — at least one source file is required");
     }
 
-    Opts { files, no_main }
+    Opts {
+        files,
+        no_main,
+        output,
+    }
 }