fix: Make the JSON tokenizer and writer RFC 8259 compliant

[keelerm84]

Port of the v3 RFC 8259 compliance work (#46) to the v4 module line. The v4
tokenizer/writer code is identical to v3's aside from the (already removed)
easyjson build tags, so this applies the same logic; the tests drop the build
tag to match v4's single-backend layout.

Reader:

• Whitespace: allow only space, tab, LF, and CR between tokens.
• Numbers: reject leading zeros, a bare minus, and a trailing decimal point;
  integers that overflow int64 fall back to float parsing so they match
  encoding/json instead of returning a wrapped value.
• Strings: reject unescaped control characters (below 0x20); substitute the
  Unicode replacement character for invalid UTF-8 bytes; combine UTF-16
  surrogate pairs into a single code point (lone/invalid surrogates decode to
  U+FFFD), matching encoding/json.

Writer:

• Float64 returns an error for NaN and infinities rather than emitting invalid
  JSON.

---

Note

Medium Risk
Changes core JSON parsing/encoding semantics and will reject or decode some inputs differently than before; behavior is heavily tested against encoding/json but any caller relying on the old lax behavior could break.

Overview
Brings the v4 jreader / jwriter default tokenizer in line with RFC 8259 and encoding/json behavior (port of prior v3 work).

Reader: Token whitespace is limited to space, tab, LF, and CR (no other Unicode spaces). Number parsing is rewritten to enforce JSON grammar (leading zeros, incomplete fractions/exponents, etc.); int64 overflow no longer wraps—literals fall through to strconv.ParseFloat like the stdlib. Strings reject unescaped control chars; invalid UTF-8 and lone/malformed \u surrogates decode like encoding/json (replacement char, surrogate-pair combining via new readUnicodeEscape).

Writer: Float64 now errors on NaN and ±Inf instead of emitting invalid JSON.

New RFC-focused test suites compare accept/reject and decoded values against encoding/json.

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