Rework BDCT into first-class BODFT primitive with iterative scheduling

[falseywinchnet]

Reframes the half-bin transform as BODFT (Bruun odd-frequency DFT), a first-class half-bin spectral primitive, and removes all DCT-I..IV framing and code. BODFT computes

H[k] = sum_n x[n] exp(-2*pi*i*(k+1/2)*n/N)

for real input -- the spectrum sampled halfway between the ordinary FFT bins -- emitting the packed lower half H[0..N/2-1].

Changes:

• Rename kernel/API bdct -> bodft (namespace bodft, bodft::plan/plan_f32, C ABI bodft_plan/bodft_forward/bodft_inverse[_f32], C++ bfft::bodft).
• Strip the DCT-IV path entirely (dctiv_plan, DCT-IV API, dctiv benchmark).
• Replace the recursion with iterative scheduling: a build-time radix-4 digit-reversal permutation drives one leaf pass, then log4(N) ping-pong combine passes run leaves-up (forward) / top-down (inverse). This also improves locality (forward ~20% faster than the recursion at large N).
• Forward and inverse are first class in both float and double. Forward uses the explicit 128-bit SoA SIMD combine (V2 double, V4F float, FMA, portable SSE+NEON); inverse is the exact algebraic inverse (scalar).
• Precomputed t, t^2, t^3 twiddle tables (three muls per combine, no derivation chain).

Validation: examples/bodft_benchmark.cpp checks both precisions across N=2..8192 -- forward vs direct DFT, exact impulse phase, exact inverse roundtrip (~1e-16 double, ~1e-7 float). Warning-clean under -Wall -Wextra -Wpedantic; existing make test suite still passes.