build(Makefile): LLD linker, objects/ dir, LTO+linker opts

- Compiler selection: Remove TCC, default to Clang, GCC fallback
- LLD linker: -fuse-ld=lld for all builds (peachwm + peachmsg)
- Object relocation: .o files output to objects/{src,parser,protocols,peachmsg}/
- Clean target: rm -rf objects + transitional stale .o cleanup
- Release: -flto=thin, linker opts (gc-sections, icf=safe, z now, etc.)
- Release: strip both peachwm and peachmsg binaries
- README + CONTRIBUTING: Add LLD dependency, update build docs
- .gitignore: Add objects/ directory

CONTRIBUTING.md

@@ -73,9 +73,11 @@ PeachWM follows a consistent style throughout the codebase. Please match it.
 ### Build and Compiler
 
 - The project uses **C23** (`-std=c23`).
-- The **Makefile** is POSIX make. Do not introduce autotools, CMake, or meson.
-- Ensure your code compiles cleanly with `make debug` and `make release` --
-  CI checks both targets.
+- The **Makefile** uses GNU make extensions (`.ONESHELL`, `ifeq`, target-specific
+  variables). Do not introduce autotools, CMake, or meson.
+- Ensure your code compiles cleanly with `make debug` and `make release` -- the
+  project requires Clang (default) and the LLD linker for building. CI checks
+  both targets.
 
 These guidelines are suggestions, not hard rules. When in doubt, match the
 style of the surrounding code.