fix: order StringType.compare by code point to match spec

[spokodev]

Problem

StringType.compare() and compareBuffers() disagree for the same string values:

const type = avsc.Type.forSchema('string');
const a = '', b = '\u{1F600}'; // U+E000 vs U+1F600

type.compare(a, b);                                       // +1  (wrong)
type.compareBuffers(type.toBuffer(a), type.toBuffer(b));  // -1  (correct)

Both are public comparison APIs, so a caller cannot rely on a single sort order.

Spec

The Avro spec Sort order defines string ordering as:

strings are compared lexicographically by Unicode code point. Note that since UTF-8 is used as the encoding for strings, sorting the bytes (which is what compareBuffers does) gives the same result as comparing the code points.

So code-point order and UTF-8 byte order are identical, and compareBuffers/_match already follow it. compare did not.

Root cause

StringType defines no compare, so it inherits PrimitiveType.prototype.compare, which is utils.compare(a, b) — a plain a < b. For strings, < compares by UTF-16 code unit, not code point. The two diverge for supplementary plane (astral) characters: a surrogate lead unit (0xD800–0xDBFF) sorts before BMP code points U+E000–U+FFFF, whereas by code point the astral character sorts after. Meanwhile StringType._match uses tap1.matchBytes(tap2) (UTF-8), so the two APIs disagree.

Fix

Add StringType.prototype.compare, walking both strings by code point. This matches _match/compareBuffers and the spec. RecordType and ArrayType compare delegate to the field comparator, so they are corrected for string fields too.

Verification

Red before, green after; added two tests in the StringType suite asserting compare agrees with compareBuffers and follows code-point order for astral characters. Full suite passes (504 tests).