build: document system deps, add check-deps, auto-glob web modules

- README: rewrite Build section for the hand-written Makefile (make /
  make DEBUG=1 / run / test / bench). Add a "System dependencies" table
  with the X11+GL dev packages per distro (apt/dnf/pacman/zypper/apk),
  since a clone won't build without them and the error is otherwise
  cryptic. Fix stale ./bin/Debug paths to ./bin/Release.
- Makefile: add `make check-deps` — probes for the required X11/GL dev
  headers and prints the install command for the detected distro on
  failure (via /etc/os-release).
- build_web.sh: auto-discover app modules from src/*.c (drop the
  hardcoded APP_MODULES list that had to be hand-synced), matching the
  desktop Makefile. Excludes the desktop platform backends. Remove the
  stale rspektrum.make reference.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -61,37 +61,67 @@ format.
 
 ## Building
 
-The build is driven by a checked-in Makefile. `premake5` is **not** required to
-build — only to regenerate the makefiles.
+You need only **`make` and a C compiler** (`gcc` or `clang`) plus the X11/OpenGL
+**development** headers (see below). raylib is vendored in this repo and compiled
+from source — there is no separate raylib install step, no `premake`, no network
+access required. A plain clone builds:
 
 ```bash
-make -f rspektrum.make config=debug_x64      # -> bin/Debug/rspektrum
-make -f rspektrum.make config=release_x64    # -> bin/Release/rspektrum
+make                # release -> bin/Release/rspektrum   (-O3 -ffast-math, AVX2/FMA)
+make DEBUG=1        # debug   -> bin/Debug/rspektrum     (-g, no optimization)
+make run            # build + launch
+make test           # build + run the DSP correctness tests
+make bench          # FFT benchmark over mlnl_samples.wav
+make clean
 ```
 
-Web (WebAssembly) build:
+Useful overrides: `make CC=clang`, or `make ARCH=-march=native` to tune for your
+own CPU (the default `-march=x86-64-v3` targets any ~2013+ x86-64 chip; drop it
+with `make ARCH=` for an older CPU).
+
+### System dependencies
+
+The compiler needs the X11 and OpenGL **dev** headers (the runtime libs are
+already present on any desktop; only the `-dev`/`-devel` packages are usually
+missing). The X11 extension libraries (Xrandr, Xinerama, Xcursor, Xi) are opened
+at runtime via `dlopen`, but their **headers** are still required to compile.
+
+If `make` stops with an error like `fatal error: X11/Xlib.h: No such file` or
+`GL/gl.h: No such file`, install the dev packages for your distribution:
+
+| Distro | Command |
+|--------|---------|
+| **Debian / Ubuntu / Mint** | `sudo apt install build-essential libx11-dev libxrandr-dev libxinerama-dev libxcursor-dev libxi-dev libgl1-mesa-dev` |
+| **Fedora / RHEL / Rocky** | `sudo dnf install gcc make libX11-devel libXrandr-devel libXinerama-devel libXcursor-devel libXi-devel mesa-libGL-devel` |
+| **Arch / Manjaro** | `sudo pacman -S base-devel libx11 libxrandr libxinerama libxcursor libxi mesa` |
+| **openSUSE** | `sudo zypper install gcc make libX11-devel libXrandr-devel libXinerama-devel libXcursor-devel libXi-devel Mesa-libGL-devel` |
+| **Alpine** | `sudo apk add build-base libx11-dev libxrandr-dev libxinerama-dev libxcursor-dev libxi-dev mesa-dev` |
+
+On Debian/Ubuntu the single metapackage `xorg-dev` pulls in all of the X11 `-dev`
+packages above, if you'd rather not list them.
+
+Run `make check-deps` to probe for the required headers before building — it
+prints the install hint for your platform if anything is missing.
+
+### Web (WebAssembly) build
 
 ```bash
-./build_web.sh      # emscripten; emits the WebAssembly bundle
+./build_web.sh      # emscripten; emits the WebAssembly bundle to bin/web/
 ```
 
-> The release build enables `-O2`, which turns on extra warnings
-> (`-Wformat-truncation`) that the debug build doesn't. Build release before
-> declaring a change clean.
-
 ---
 
 ## Usage (desktop GUI)
 
 ```bash
-./bin/Debug/rspektrum [input.wav]
+./bin/Release/rspektrum [input.wav]
 ```
 
 Load a file by passing it on the command line, dragging a `.wav` onto the window,
 or pressing **O** for the file browser. Try the bundled sample:
 
 ```bash
-./bin/Debug/rspektrum mlnl_samples.wav      # in-repo WAV with an embedded mLnL chunk
+./bin/Release/rspektrum mlnl_samples.wav    # in-repo WAV with an embedded mLnL chunk
 ```
 
 ### Controls