Schema3 and parsing fixes

[michalz-zoom]

• Fix hex decoding of trailing junk: decode_hex rejects invalid/trailing junk instead of Buffer.from(..., 'hex') silently truncating; used by is_hex, unhex, and schema binary convert.
• Fix Or combinator in schema3: check() returns an error or null, so the loop was inverted; matching terms are accepted and the WoT bad-proof case is covered.
• Return errors for null schema values: _check_value now returns the null error instead of dropping it, so required fields reject null/undefined.
• Reject __proto__ keys and use own-property lookups in schema3: lookups use hasOwnProperty.call; __proto__ is banned as a schema, payload, and enum key.