fix: emit warning for unknown preprocessor directives

[liuxiaopai-ai]

Summary

Emit a warning diagnostic when the preprocessor encounters an unknown directive (e.g. #defin, #inclde). Previously these were silently ignored, which made typos in preprocessor directives very hard to catch.

Fixes #172

Changes

• src/frontend/preprocessor/pipeline.rs: In the process_directive() catch-all arm, instead of silently ignoring unknown directives, push a PreprocessorDiagnostic warning with the message unknown preprocessing directive #<keyword>.
• Added 6 unit tests covering:
  • Unknown directive (#defin) emits a warning
  • Another typo (#inclde) emits a warning
  • Known directives (#define, #undef, #ifdef, #endif) produce no warnings
  • Null directive (# alone) produces no warning (valid per C standard)
  • Unknown directives inside inactive conditional blocks (#if 0) do not warn
  • Correct source location (file, line) is reported in the diagnostic

Behavior

This matches GCC and Clang behavior, which both emit a warning (or error) for unknown preprocessing directives:

test.c:1:2: warning: unknown preprocessing directive #defin