fix: escape string values for date/date-time/time formats

[mcollina]

Problem

asDateTime, asDate and asTime concatenate string inputs straight between quotes:

if (typeof date === 'string') {
  return '"' + date + '"'
}

So a string value containing ", \, a control character or a lone surrogate produces output that is not valid JSON:

const stringify = fastJson({
  type: 'object',
  properties: { ts: { type: 'string', format: 'date-time' } }
})

stringify({ ts: 'a"b' })   // {"ts":"a"b"}  -> JSON.parse throws

Plain type: string fields already go through asString, so whether a string got escaped depended on format, which is surprising.

Fix

• Hoist the escaping routine to module scope as asString(str); the class method delegates to it. This is needed because the generated code destructures the serializer methods and calls them unbound, so this.asString isn't reachable from asDateTime & co.
• Route the string branch of asDateTime / asDate / asTime through it.
• Date inputs keep the plain concatenation — toISOString() output can never need escaping.
• format: 'unsafe' is untouched; it remains the documented way to opt out of escaping.

Notes

• Serializing a date-format string now costs the same as serializing a regular string of the same length (roughly 10ns -> 37ns per field in a tight loop for a 24-char ISO timestamp). Unavoidable if the output has to be valid JSON. Date values are unaffected.
• Values are still not reformatted or validated as dates, only escaped. README note updated to say so.

Test

New case in test/date.test.js asserts all three formats round-trip quotes, backslashes, control characters and lone surrogates identically to JSON.stringify.

🤖 Generated with Claude Code

https://claude.ai/code/session_01TmhA1oTx5qUWc6UmEgWuar

[mcollina]

The real reason for this is so we stop receiving reports.