Add variant option (atomify wasm in the kernel) and ship python/ in the npm tarball

[andeplane]

Two coupled changes that Atomify needs from lammps.js.

1. variant: "serial" | "kokkos" | "atomify" client option

The full-package atomify wasm build already ships in the npm tarball (./wasm-atomify, built by PACKAGES=atomify python3 cpp/build.py), but neither LammpsClient nor the Python bindings could load it — so the Pyodide kernel was stuck on the MOLECULE-only serial build.

• client.ts: variant selects the wasm module (lammps.js, lammps-kokkos.js, or lammps-atomify.js). Precedence: an explicit variant wins over the kokkos flag; kokkos: true remains equivalent to variant: "kokkos" (full back-compat). Since the atomify build is itself a KOKKOS/pthreads build, both threaded variants start LAMMPS with -k on t N [-sf kk] (threads/suffix still tunable via the kokkos option) and require a cross-origin isolated context in browsers.
• worker path: variant is forwarded through LammpsWorkerClient, the worker init protocol message, and the worker host.
• python/lammps/init.py: await lammps(variant="atomify") mirrors the client option. The crossOriginIsolated check now covers every threaded variant, and the shared wasm-module cache is keyed per (client URL, variant) so serial/kokkos/atomify sessions don't collide. Reused sessions now also receive the client options, so threaded restarts keep their Kokkos start args.
• types/client.d.ts: declares KokkosOptions, LammpsVariant, and the new options.
• docs: README, python/README, and NOTEBOOK_TUTORIALS updated; the "no LammpsClient entry point for the atomify variant" caveat is gone.

2. Ship python/ in the npm tarball + version alignment

python/lammps, python/pyproject.toml, and python/README.md are added to the package files, so consumers can build the lammps-js wheel directly from node_modules/lammps.js/python without cloning this repo. The entries are listed individually so local build artifacts (python/build/, python/lammps_js.egg-info/) can never leak into the tarball. The wheel version is bumped 1.5.0 → 1.5.1 to match the npm package version exactly.

Why Atomify needs this

Atomify's JupyterLite kernel runs the lammps.js Python bindings on top of the wasm engine. Its example library needs the full package set (MANYBODY, KSPACE, REAXFF, GRANULAR, …), which only the atomify build provides — variant="atomify" lets the kernel run it. Shipping python/ in the tarball lets Atomify CI build the wheel from node_modules instead of vendoring it. After this merges and a release is cut, Atomify can drop its vendored-wheel version pin.

Tests

• Full vitest suite green locally against a freshly built serial wasm: 146 passed, 12 skipped (the kokkos/atomify specs skip where those builds are absent; CI covers them via test:kokkos / test:atomify).
• New tests: variant precedence + Kokkos start args (client.unit.spec.ts), init-message forwarding (worker-host.spec.ts), and an end-to-end variant: "atomify" client test in atomify.spec.ts.
• npm pack --dry-run verified: the tarball gains exactly python/lammps/__init__.py, python/pyproject.toml, python/README.md.

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