feat(f3dock-nfft): add FastGaussianSum wrapper library with direct-evaluator reference (Phase 5 task 2)

[transfix]

Summary

Adds a new static library f3dock-nfft exposing f3dock::nfft::FastGaussianSum, a thin wrapper that evaluates a 3D Gaussian sum

$$f(y_j) = \sum_{k=0}^{N-1} \alpha_k \cdot \exp(-c \cdot |y_j - x_k|^2)$$

at a batch of target points {y_j}. This is the building block that Phase 5 task 3 will route the F2Dock electrostatics gridding through under the nfftAccelerate opt-in flag landed in #31.

Two evaluation paths

• evaluate_direct(out) — always-available O(N*M) reference loop. Used as ground truth in unit tests.
• evaluate(out) — selects the NFFT3 fastsum fast path when is_accelerated() returns true; otherwise delegates to the direct evaluator. The NFFT3 hookup is intentionally a no-op fallback in this PR (so the library compiles and runs identically on every platform in the CI matrix) and lands in a follow-up patch once libnfft3 is available in CI.

Build / test plumbing

• src/f3dock-nfft/CMakeLists.txt builds the static library and links the optional f3dock_science_stack INTERFACE target when present, so F2DOCK_HAVE_NFFT (and any related include/link plumbing) propagates automatically once libnfft3 is discovered.
• Library is built under the existing F2DOCK_ENABLE_F3DOCK_EXPERIMENTAL gate alongside f3dock-domain / f3dock-flex / f3dock-icp / f3dock-loop-closure.
• 7 new gtest cases in tests/unit/test_f3dock_nfft_fast_gaussian_sum.cpp lock in:
  • constructor validation (positive c, non-null sources)
  • direct evaluator vs a hand-written reference
  • evaluate() == evaluate_direct() bit-for-bit when unaccelerated
  • single-point identity bound
  • empty-source short-circuit
  • inspector accessors

Verification

• cmake --build — green on WSL Ubuntu-24.04 / gcc-13
• 197/197 unit tests pass (190 previous + 7 new)
• clang-format-18 --Werror clean

Follow-ups

• PR C (task 3): wire FastGaussianSum into the electrostatics gridding path in src/fft-utils/fastfft.cpp::gridding() under the nfftAccelerate switch; add integration smoke baseline.
• PR D (task 4): libcvc geometry consumer audit (Blurmaps/GOA/PRGN).
• Future: real NFFT3 fastsum hookup in evaluate() (requires libnfft3 in CI runners).