chore(node, readme): SPDX license, Node 18 engines, bench hardware spec (R15, R18, R19)

R15 — `bindings/node/package.json` and all six per-platform subpackage
templates (`npm/{darwin-arm64,darwin-x64,linux-arm64-gnu,linux-x64-gnu,
win32-arm64-msvc,win32-x64-msvc}/package.json`) plus the WASM
`release.yml` enrich step switch the `license` field from the npm
convention `SEE LICENSE IN LICENSE` to the SPDX identifier
`PolyForm-Noncommercial-1.0.0`. npm's license search and downstream
tooling can now surface the actual license.

R18 — every `engines.node` field is bumped from `>= 16` to `>= 18`.
The package's test script (`node --test __tests__/`) uses the built-in
`node --test` runner that landed in Node 18; advertising support for
Node 16 / 17 was a guarantee we never verified (CI tests on Node 18 /
20 only) and would have produced a confusing error on those versions.

R19 — README's benchmark section gains an explicit hardware /
software-version block and reframes the absolute µs values as a
relative-speedup snapshot rather than a universal contract. Also tells
the reader how to reproduce locally (`pip install -e
bindings/python[bench]` + `python -m benchmarks.compare_libraries`) and
where the CI artefact lives.

Side effects:

- R16 / R17 (built wheel under `bindings/python/dist/` and
  `examples/node/node_modules/`) — verified that both are already
  covered by `.gitignore` (`*.whl`, `dist/`, `**/node_modules/`) and
  were never tracked by git. The local directories have been cleared;
  no `.gitignore` change needed and no committed file removed.

README.md

@@ -52,7 +52,21 @@ multi-language reach, and active maintenance.
 
 ## Benchmark: how much faster is "streaming-first"?
 
-Reproduced on this machine with `python -m benchmarks.compare_libraries`.
+The numbers below were measured on a single developer workstation and are not
+guaranteed to reproduce identically on different hardware — absolute µs values
+depend on CPU, memory clock and OS scheduler. Read them as **relative
+speedups** between libraries on identical input, not as a universal
+performance contract.
+
+- **Reproduced on:** Windows 11 Pro 26200, AMD Ryzen 9 7950X3D, 64 GB DDR5,
+  Rust 1.92 (release profile, `lto = "fat"`, `codegen-units = 1`),
+  Python 3.12, Node 20.
+- **Reproduce yourself:** `pip install -e bindings/python[bench]` then
+  `python -m benchmarks.compare_libraries`. The script auto-detects every
+  installed peer library and runs them on the same generated inputs as
+  Wickra. The CI job `cross-library-bench` runs the same script on every
+  push and uploads the raw report as a build artefact.
+
 Lower µs/op = faster. Wickra wins every batch category outright, and the
 streaming gap widens linearly with how much history a batch-only library has
 to recompute on every tick.