feat(pic1d): add 1D electrostatic PIC solver

[joglekara]

Summary

New solver: pic-1d that reuses Vlasov-1D's input deck (units, density profiles, drivers, save schema) so PIC and Vlasov can be compared on the same configs. Adds particle-specific knobs grid.ppc, grid.particle_shape, terms.time, and per-species loading.

Solver

• B-spline (linear / tsc / cubic) deposit & gather, spectral Poisson on the Vlasov grid, KDK leapfrog + 4th-order Yoshida symplectic compositions.
• PIC1DVectorField wraps it for diffrax with the same dummy Euler Stepper Vlasov-1D uses.
• Transverse light wave: when drivers.ey is configured, the same step also advances a transverse vector potential a(x) via the Vlasov-1D wave solver (with the matching 2nd-order ABC), evaluates the TransverseCurrentSourceDriver source (extended or point), and adds the ponderomotive force F_pond = -½ ∂_x(a²) to each species' kick. Particles remain 1D1V. Pure-ES runs (empty ey) skip the wave solve entirely.
• Quasineutrality: density.quasineutrality: true auto-balances the ion background as -Σ_s q_s n_s, so multi-species electron-only setups (e.g. two cold beams) work without manual ion configuration.

Post-process

• Storage saves nested per-species moments {n, j, P} computed via the same deposit kernel as the field solver, plus shared e, de, a, prev_a, pond.
• Plot tree mirrors Vlasov-1D: linear + log10 spacetime and facet lineouts per field, plots/fields/<species>/… and plots/scalars/<species>/… subtrees, and ep/em decomposition when an ey driver is configured.
• Dist saves write raw particle (x, v) snapshots and phase-space scatter plots (no histogram binning into f).

Tests (tests/test_pic1d/)

• test_two_stream — seeded cold counter-streaming beams, log-linear fit of γ against ωp/(2√2) for TSC and cubic shapes (~2 s).
• test_bohm_gross — quietly-loaded Maxwellian at k=0.3, free oscillation, time-FFT peak ω vs the exact kinetic Langmuir dispersion (real root of Re ε via the plasma dispersion function) for both leapfrog and yoshida4 (~3 s).
• test_landau_damping — driver + noise-subtraction at 1M PPC; marked slow (~6 min wall clock) with factor-of-2 tolerance on the Landau rate.

Configs

• configs/pic-1d/epw.yaml — direct port of the Vlasov-1D EPW config.
• configs/pic-1d/two-stream.yaml — cold equal-beam two-stream setup.

Lint

• Extends ruff.toml's allowed-confusables list with −, ×, γ so physics-notation docstrings/assertions pass RUF001/RUF002.

Test plan

• uv run pytest tests/test_pic1d/test_two_stream.py tests/test_pic1d/test_bohm_gross.py
• uv run pre-commit run --files <changed files>
• uv run run.py --cfg configs/pic-1d/epw.yaml (full lifecycle through ergoExo + post-process tree)
• uv run run.py --cfg configs/pic-1d/two-stream.yaml
• uv run pytest tests/test_pic1d/test_landau_damping.py -m slow (long; verified the underlying 1M-PPC run manually but not under pytest yet)

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