build: replace premake with a hand-written Makefile; optimize FFT

Build system
- Add a single hand-written Makefile (GNU make + gcc, pure C). Builds
  raylib from vendored source and links rspektrum with no premake/lua.
  Targets: all/run/test/bench/clean; release default, DEBUG=1 for debug;
  ARCH overridable (defaults to -march=x86-64-v3).
- Remove premake entirely: rspektrum.make, raylib.make, build/premake5*
  binaries, build/premake5.lua, build/ecc/*. The generated top-level
  Makefile was gitignored, so hand-edits to it were silently lost.
- Vendor raylib src/ into the repo (was gitignored -> fresh clones could
  not build). Commit only src/ (~16MB); examples/projects stay local.
  Verified: a build from the git-tracked tree alone succeeds offline.
- Release flags bumped to -O3 -ffast-math with a portable arch baseline
  (x86-64-v3 = AVX2+FMA on x64, SSE2 on x86). Confirmed FMA/AVX codegen
  in fft.o.

FFT optimization (src/fft.c)
- Precompute twiddle factors and the bit-reversal permutation once per
  size, cached as a small plan table (FFTW's idea, lightweight). Removes
  the per-butterfly cexpf() and per-element bit-twiddling that dominated.
- 3.6x faster on the mlnl_samples.wav STFT workload (2048-pt, -O2 same
  flags both sides): 81us -> 22us per FFT. With the new -O3/-ffast-math/
  AVX2 release flags stacked: ~15us (5.5x vs the old -O2 baseline).
- Verified vs a double-precision reference DFT: 1e-6 relative error,
  round-trip 2.4e-7. Drop-in: same FFT() signature.

Tests/bench (bench/)
- fft_verify.c: FFT vs reference DFT + round-trip check (make test).
- fft_bench.c: times the real STFT workload (make bench).

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

Makefile

@@ -0,0 +1,110 @@
+# rspektrum - hand-written Makefile (GNU make + gcc, no premake/lua).
+#
+# Quick start:
+#   make            # release build  -> bin/Release/rspektrum
+#   make DEBUG=1    # debug build    -> bin/Debug/rspektrum
+#   make run        # build + launch
+#   make test       # build + run the DSP correctness tests
+#   make bench      # build + run the FFT benchmark over mlnl_samples.wav
+#   make clean      # remove build artifacts for the current config
+#
+# Override the toolchain or flags on the command line, e.g.
+#   make CC=clang
+#   make ARCH="-march=native"     # bleeding-edge tune for your own machine
+#
+# Pure C project (raylib is vendored as source and compiled in). Linux/X11.
+
+# ---- toolchain ----------------------------------------------------------
+CC      ?= gcc
+AR      ?= ar
+
+# ---- config: release (default) or DEBUG=1 -------------------------------
+# ARCH is the portable baseline tune; override to -march=native for a local
+# build. x86-64-v3 == AVX2 + FMA (~2013+), the sweet spot for the float FFT.
+ARCH    ?= -march=x86-64-v3
+
+ifdef DEBUG
+  BUILD     := Debug
+  OPT       := -g
+  DEFCONFIG := -DDEBUG
+else
+  BUILD     := Release
+  OPT       := -O3 -ffast-math $(ARCH)
+  DEFCONFIG := -DNDEBUG
+  STRIP     := -s
+endif
+
+BINDIR := bin/$(BUILD)
+OBJDIR := obj/$(BUILD)
+TARGET := $(BINDIR)/rspektrum
+RAYLIB := $(BINDIR)/libraylib.a
+
+# ---- shared flags -------------------------------------------------------
+RAYLIB_SRC := build/external/raylib-master/src
+DEFINES    := $(DEFCONFIG) -DPLATFORM_DESKTOP -DGRAPHICS_API_OPENGL_33 -D_GLFW_X11
+INCLUDES   := -Isrc -Iinclude -I$(RAYLIB_SRC) -I$(RAYLIB_SRC)/external/glfw/include
+CFLAGS     := -std=c17 -Wall -Wshadow $(OPT) $(DEFINES) $(INCLUDES)
+DEPFLAGS    = -MMD -MP
+LDLIBS     := -lpthread -lm -ldl -lrt -lX11
+LDFLAGS    := $(STRIP)
+
+# ---- sources ------------------------------------------------------------
+# App: every src/*.c except the non-Linux platform backends.
+APP_SRC := $(filter-out src/platform_win32.c src/platform_web.c,$(wildcard src/*.c))
+APP_OBJ := $(patsubst src/%.c,$(OBJDIR)/app/%.o,$(APP_SRC))
+
+# raylib: explicit list of the modules we link (matches the vendored tree).
+RAY_MODULES := rcore rshapes rtextures rtext rmodels raudio rglfw
+RAY_OBJ     := $(patsubst %,$(OBJDIR)/raylib/%.o,$(RAY_MODULES))
+
+DEPS := $(APP_OBJ:.o=.d) $(RAY_OBJ:.o=.d)
+
+# ---- top-level targets --------------------------------------------------
+.PHONY: all run test bench clean help
+.DEFAULT_GOAL := all
+
+all: $(TARGET)
+
+$(TARGET): $(APP_OBJ) $(RAYLIB) | $(BINDIR)
+	@echo "Linking $@"
+	$(CC) $(LDFLAGS) -o $@ $(APP_OBJ) $(RAYLIB) $(LDLIBS)
+
+$(RAYLIB): $(RAY_OBJ) | $(BINDIR)
+	@echo "Archiving $@"
+	$(AR) rcs $@ $(RAY_OBJ)
+
+# ---- compile rules ------------------------------------------------------
+$(OBJDIR)/app/%.o: src/%.c | $(OBJDIR)/app
+	@echo "  CC $<"
+	$(CC) $(CFLAGS) $(DEPFLAGS) -c $< -o $@
+
+$(OBJDIR)/raylib/%.o: $(RAYLIB_SRC)/%.c | $(OBJDIR)/raylib
+	@echo "  CC $<"
+	$(CC) $(CFLAGS) $(DEPFLAGS) -c $< -o $@
+
+# ---- dirs ---------------------------------------------------------------
+$(BINDIR) $(OBJDIR)/app $(OBJDIR)/raylib:
+	@mkdir -p $@
+
+# ---- convenience --------------------------------------------------------
+run: $(TARGET)
+	./$(TARGET)
+
+# DSP tests/benchmarks (fft.c is standalone, so they link it directly with the
+# same release flags this Makefile uses).
+test:
+	$(MAKE) -C bench test CC="$(CC)" CFLAGS="-O3 -ffast-math $(ARCH) -Wall"
+
+bench:
+	$(MAKE) -C bench bench CC="$(CC)" CFLAGS="-O3 -ffast-math $(ARCH) -Wall"
+
+clean:
+	rm -rf $(OBJDIR) $(TARGET) $(RAYLIB)
+	$(MAKE) -C bench clean
+
+help:
+	@echo "Targets: all (default), run, test, bench, clean"
+	@echo "Configs: release (default), or 'make DEBUG=1'"
+	@echo "Vars:    CC, AR, ARCH (e.g. ARCH=-march=native)"
+
+-include $(DEPS)