The JSON lexer iterated over raw bytes and cast each byte directly to
char, which corrupts multi-byte UTF-8 sequences (e.g. "café" would
become "café"). Now properly detects UTF-8 lead bytes and reads the
full multi-byte sequence before pushing to the output string.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

## Math
- Not a constructor (static methods only)
- Constants: E, LN2, LN10, LOG2E, LOG10E, PI, SQRT1_2, SQRT2
- Methods: abs, ceil, floor, round, trunc, max, min, pow, sqrt, cbrt,
hypot, sin, cos, tan, asin, acos, atan, atan2, exp, log, log2, log10,
expm1, log1p, sign, clz32, fround, random, imul

## Date
- Constructor: Date(), Date(ms), Date(string), Date(y, m, d, h, min, s, ms)
- Static: Date.now(), Date.parse(string), Date.UTC(...)
- Getters: getFullYear, getMonth, getDate, getDay, getHours, getMinutes,
getSeconds, getMilliseconds, getTime, getTimezoneOffset + UTC variants
- Setters: setFullYear, setMonth, setDate, setHours, setMinutes,
setSeconds, setMilliseconds, setTime
- Formatting: toString, toISOString, toUTCString, toLocaleDateString, toJSON
- valueOf returns milliseconds since epoch

## JSON
- JSON.parse(text): full JSON tokenizer and recursive descent parser
supporting objects, arrays, strings, numbers, booleans, null, and
escape sequences including \uXXXX unicode escapes
- JSON.stringify(value, replacer?, space?): recursive serialization with
pretty-print support, circular reference detection (throws TypeError),
handles NaN/Infinity as null, Date toJSON support

## VM Changes
- Added date_prototype field to Vm
- Added RuntimeError::syntax_error constructor
- Made js_number_to_string pub(crate) for JSON.stringify

41 new tests (369 total JS tests, all passing)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

## String
- Constructor: String(), String.fromCharCode(), String.fromCodePoint()
- Prototype: charAt, charCodeAt, codePointAt, concat, slice, substring,
substr, indexOf, lastIndexOf, includes, startsWith, endsWith, trim,
trimStart, trimEnd, padStart, padEnd, repeat, split, replace,
replaceAll, toLowerCase, toUpperCase, at, toString, valueOf

## Number
- Constructor: Number()
- Static: isNaN, isFinite, isInteger, isSafeInteger, parseInt, parseFloat
- Constants: EPSILON, MAX_SAFE_INTEGER, MIN_SAFE_INTEGER, MAX_VALUE,
MIN_VALUE, NaN, POSITIVE_INFINITY, NEGATIVE_INFINITY
- Prototype: toString(radix), valueOf, toFixed, toPrecision, toExponential

## Boolean
- Constructor: Boolean() with truthiness coercion
- Prototype: toString, valueOf

## Symbol
- Factory: Symbol(description) producing unique string-based identifiers
- Well-known: Symbol.iterator, toPrimitive, toStringTag, hasInstance
- Registry: Symbol.for(key), Symbol.keyFor(sym)

## VM Changes
- Added string_prototype, number_prototype, boolean_prototype to Vm
- Primitive auto-boxing: GetProperty/GetPropertyByName now look up
prototype methods for String, Number, and Boolean values
- Global constants: NaN, Infinity, undefined

37 new tests (328 total JS tests, all passing)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>