ci+loader: build with --platform; ship napi-generated loader

The Windows Node 18 and Node 20 CI jobs failed because:

- ci.yml ran 'napi build --release' (no --platform), which produces a
  filename without the target triple ('wickra.node').
- The committed loader at bindings/node/index.js looked for the
  triple-suffixed file ('wickra.win32-x64-msvc.node') and then for the
  per-platform npm subpackage as a fallback.
- 'wickra-win32-x64-msvc' isn't on npm yet (blocked by the spam filter),
  so Windows had nothing to load. Linux and macOS passed only because
  their optionalDependencies do exist on npm and got fetched.

Two-part fix:

- ci.yml now runs 'napi build --platform --release', matching release.yml.
  --platform encodes the target triple in the filename, so the loader's
  primary lookup always finds the freshly built binary on every host.

- bindings/node/index.js is now the canonical napi-rs auto-generated
  loader (it gets regenerated by 'napi build'). It covers all the platforms
  napi knows about (linux glibc/musl, Android, FreeBSD, ARM, etc.) rather
  than just the four we publish, so unsupported platforms get a clear
  'this binding isn't built for your system' error instead of a confusing
  ENOENT.

Local sanity: 'npx napi build --platform --release' then 'node --test
__tests__/' succeeds end to end on Windows; the eight existing tests
pass.

.github/workflows/ci.yml

@@ -146,7 +146,11 @@ jobs:
 
       - name: Build native module
         working-directory: bindings/node
-        run: npx napi build --release
+        # --platform puts the target triple into the filename so the loader's
+        # `wickra.<target>.node` lookup finds the freshly built binary instead
+        # of falling back to the per-platform npm subpackage (which doesn't
+        # exist yet for win32-x64-msvc).
+        run: npx napi build --platform --release
 
       - name: Run Node tests
         working-directory: bindings/node