Add standalone time-only ISO-8601 format support (Clause 5.4)

[Copilot]

ISO-8601 Clause 5.4 defines standalone time representations without date components (10:30, 10:30:45, 10:30:45.123). Parser currently requires a date component, rejecting valid time-only inputs.

Changes

Type system (parser-types.ts)

• Added TimeAst to ValueAst union alongside DateTimeAst and DurationAst

Parser (parser.ts)

• Detect standalone time via lookahead: Number followed by Colon
• Route to existing parseTime() before attempting date parse

Stringify (stringify.ts)

• Handle TimeAst in stringifyTemporal() and stringifyRange()

Usage

// Standalone time parsing
parseTemporal('10:30')           // { kind: 'Time', hour: 10, minute: 30 }
parseTemporal('10:30:45.123')    // { kind: 'Time', ..., fraction: '123' }
parseTemporal('10:30:45,123')    // European format, normalized to dot

// Time ranges
parseTemporal('10:00/12:00')     // { kind: 'Range', start: {...}, end: {...} }

// Round-trip
stringifyTemporal(parseTemporal('10:30:45'))  // '10:30:45'

Backward compatible—existing datetime, duration, and range parsing unchanged.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>bug: Support time only ISO-8601 format</issue_title>
<issue_description>particularly

• A time of day may be represented alone, without any date component.

Clause 5.4 defines formats such as:

• hh:mm:ss
• hh:mm:ss.sss
• Reduced precision forms like hh:mm

These are all valid standalone time representations.

Represent as a standalone TimeAst</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes bug: Support time only ISO-8601 format #13

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