Add float32 Opus encoding

[biw]

fixes: discordjs/opus#171

Summary

• Add a safe encodeFloat(Buffer) API backed by opus_encode_float.
• Validate float PCM type, alignment-independent byte layout, finite samples, and supported Opus frame durations.
• Document the public API and add float plus legacy int16 codec regression coverage.

Validation

• pnpm exec vp check --fix
• pnpm exec node-gyp rebuild
• LIBOPUS_TEST_NATIVE=1 node src/tests/test.js
• pnpm build-package && pnpm test && pnpm check
• git diff --check