[Aschenbath]

Add isISO8601Duration(str) so applications can validate media lengths and other ISO durations without supplying their own regular expression. The existing date-only isISO8601 behavior is unchanged.

Supports ordered year/month/day/time designators, standalone weeks, and alternative calendar/ordinal representations in basic or extended form, including date-only/time-only forms and reduced time precision. Fractions use a dot or comma and are restricted to the last represented unit. Alternative field limits follow ISO 8601:2004 section 4.4.3.3; ordinal days use the explicitly documented range 000 through 366 without leap-year checks. Signed durations, mixed weeks, expanded years and intervals are outside this API's documented scope.

Closes #2379.

References: ISO 8601:2004, sections 4.4.3.2 through 4.4.3.3 and their referenced date/time formats (4.1.2, 4.1.3, 4.2.2.2 through 4.2.2.5, 4.3.2 through 4.3.3). The alternative representation describes quantities rather than calendar dates, permits zero fields, and uses inclusive carry-over limits.

Validation: npm test passes all 333 tests, including every distribution build and ESLint. The 10 new groups fail on the exact base because the API is absent. Packed CommonJS, direct-module, browser, minified-browser and native ES-module consumption pass all 263 fixture inputs; Node 8 also passes the CommonJS/browser checks. Independent review passed 12,667 generated/boundary cases across four distributions (50,668 assertions). The direct new module also passes on Node 0.10; loading the complete package on that version encounters an existing Set dependency, reproduced on the exact base.

Implemented with OpenAI Codex and independently reviewed and tested by a separate Codex agent. No human code review is claimed.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9ff3424) to head (853b075).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2956   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files          114       115    +1     
  Lines         2599      2624   +25     
  Branches       658       667    +9     
=========================================
+ Hits          2599      2624   +25     

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