feat: replace Cranelift JIT with C backend

Remove Cranelift JIT compiler and expose the existing C backend as the
compilation target. Generated C code can be compiled with GCC/Clang.

Changes:
- Remove cranelift-* dependencies from Cargo.toml
- Delete src/compiler.rs (565 lines of Cranelift code)
- Add compile_to_c() function with -o and --run flags
- Fix C backend name mangling (main -> main_lux) to avoid conflicts
- Update CLI help text and documentation

Usage:
  lux compile <file.lux>           # Output C to stdout
  lux compile <file.lux> -o out.c  # Write to file
  lux compile <file.lux> --run     # Compile and execute

C backend supports: functions, basic types, operators, if/then/else,
records, enums, Console.print. Future work: closures, lists, patterns.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/OVERVIEW.md

@@ -181,7 +181,7 @@ fn processAny(x: Int @latest): Int = x  // any version
 
 ### Planned (Not Yet Fully Implemented)
 
-- **Full Compilation**: JIT works for numeric code, strings/lists/ADTs missing
+- **Full C Backend**: Basic functions work, closures/lists/pattern variables pending
 - **Auto-migration Generation**: Migration bodies stored, execution pending
 
 ---
@@ -234,7 +234,7 @@ Quick iteration with type inference and a REPL.
 
 | Limitation | Description |
 |------------|-------------|
-| **Limited JIT** | Cranelift JIT works for numeric code only |
+| **Limited C Backend** | Basic functions work, closures/lists pending |
 | **No Package Manager** | Can't share/publish packages yet |
 | **New Paradigm** | Effects require learning new concepts |
 | **Small Ecosystem** | No community packages yet |
@@ -297,7 +297,7 @@ Quick iteration with type inference and a REPL.
 ### Not a Good Fit (Yet)
 
 - Large production applications (early stage)
-- Performance-critical code (JIT limited to numeric)
+- Performance-critical code (C backend still basic)
 - Web frontend development (no JS compilation)
 - Systems programming (no low-level control)
 
@@ -348,13 +348,12 @@ Source Code
     ├─────────────────────────┐
     ▼                         ▼
 ┌─────────────┐       ┌──────────────┐
-│ Interpreter │       │ JIT Compiler │
-│ (default)   │       │ (Cranelift)  │
+│ Interpreter │       │  C Backend   │
+│ (default)   │       │  (compile)   │
 └─────────────┘       └──────────────┘
     │                         │
     ▼                         ▼
-Values + Effects        Native Code
-                       (~160x speedup)
+Values + Effects        C Code → GCC/Clang
 ```
 
 ---
@@ -370,12 +369,12 @@ Values + Effects        Native Code
 - ✅ HTTP Server Effect (listen, accept, respond, stop)
 - ✅ Test Effect (native testing framework)
 - ✅ Console.readLine and Console.readInt
-- ✅ JIT Compiler for numeric code (~160x speedup)
+- ✅ C Backend (basic functions, Console.print)
 - ✅ Watch mode / hot reload
 - ✅ Formatter
 
 **In Progress:**
-1. **Behavioral Type Verification** - Total, idempotent, deterministic checking
+1. **C Backend Extensions** - Closures, lists, pattern variable binding
 2. **Schema Evolution** - Type system integration, auto-migration
 3. **Error Message Quality** - Context lines shown, suggestions partial
 
@@ -383,4 +382,4 @@ Values + Effects        Native Code
 4. **SQL Effect** - Database access
 5. **Package Manager** - Share code (manifest parsing exists)
 6. **JavaScript Backend** - Run in browsers
-7. **Full JIT** - Strings, lists, ADTs, effects
+7. **Behavioral Type Verification** - Total, idempotent, deterministic checking