Is my note cache encrypted? Could someone reading my hard drive see my handwriting?

Yes, the cached note files are encrypted at rest. Each cached .note and .pdf in the app's "notes" folder is sealed with AES-256 encryption, and the master key that unlocks them is itself sealed in the macOS Keychain. Someone who copies those files off your disk (a stolen laptop, another user account on the same Mac) can't read your handwriting without your Keychain. (It does NOT protect against someone already logged in as you running code as you — that's outside what at-rest encryption can do. And the separate PDFs you save in ~/Documents/Supernote are intentionally plain, openable files, so those are not encrypted.)