Problem

In `src/frontend/preprocessor/pipeline.rs` (lines 882-889), when a directive keyword is not recognized, CCC silently ignores it. GCC emits a warning for unknown/unsupported directives. Silently ignoring can mask bugs from typos.

```c
#defin FOO 1 // Typo — should be #define
int x = FOO; // FOO is undefined — wrong compilation
// GCC: warning: unknown directive
// CCC: silent
```

Files to modify

• `src/frontend/preprocessor/pipeline.rs` — Emit a warning for unrecognized directives

Acceptance criteria

• Unknown directives produce a warning
• Known directives (`#define`, `#include`, etc.) unaffected
• `cargo test --lib` passes