v0.2.0: topology, Drude weight, verified device partitioning

Three new capability areas, each pinned to exact anchors (40 tests total, all closed-form or machine-precision):

Topology (new module berry): Wilson-loop Berry phases, lattice Berry curvature and Chern numbers in the gauge-invariant formulation of Fukui, Hatsugai and Suzuki (J. Phys. Soc. Jpn. 74, 1674 (2005)). Validated on the Haldane model's known phase diagram (Haldane, PRL 61, 2015 (1988)): Chern number an exact integer to 1e-12, +/-1 inside the topological phase, 0 outside, sign reversal with flux direction, zero total over all bands; the SSH chain's Zak phase is quantized and its two dimerizations differ by exactly pi. New lattice builders ssh and haldane. Orthogonal bases only for now; models with overlap are refused explicitly.

Intraband optics (drude_weight): the Drude weight from the same nonorthogonal velocity operator as the interband conductivity, exactly invariant under a shift of the energy zero, pinned to the closed form 8 spin |t| a of the half-filled chain. sigma_optical gains a Lorentzian lineshape option, validated on the exact molecular peak height.

Verified device partitioning (principal_layers): a finite model is cut into NEGF principal layers automatically, reproducing hand-built blocks exactly and the single-impurity transmission closed form end to end - and a layer width smaller than the interaction range is refused rather than silently truncated.

Also: k_path band-structure path helper, DOI badge (Zenodo concept DOI 10.5281/zenodo.22311381), README updated with the new validation anchors and honest scope.