Preserve sitemap and page publication dates

[PrzemyslawKlys]

Source discovery needs reliable dates before it selects or orders pages. This adds sitemap entries with modification dates while preserving document order and keeping child sitemaps distinct from page URLs. The existing URL-only API shares that parser and excludes image-extension locations.

HtmlPublicationDates.Parse reads declared publication and modification dates from page metadata and matching Article/WebPage JSON-LD. It keeps those dates separate, rejects other-page identities, and retains unambiguous dates across matching graph records. Structured crawler metadata preserves these dates after article selection removes the document head.

A private downstream consumer uses the APIs for bounded discovery and source chronology. Its package repin follows the HtmlTinkerX release containing these changes.

Validation: 17 discovery-parser tests and 29 publication/structured-metadata tests pass on each of net472, net8.0, and net10.0. Independent read-only reviews covered both contracts; all actionable findings were fixed and confirmed. Downstream selector integration and the full 122-test consumer suite pass against a local candidate package. A live source check also confirms that language-query URLs retain dates declared for their canonical article URLs. Repository CI runs on this PR.

[codecov]

Codecov Report

❌ Patch coverage is 80.64516% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.19%. Comparing base (0b55f02) to head (729e322).

Files with missing lines	Patch %	Lines
Sources/HtmlTinkerX/HtmlPublicationDates.cs	80.00%	0 Missing and 8 partials ⚠️
Sources/HtmlTinkerX/HtmlDiscoveryParser.cs	81.25%	1 Missing and 2 partials ⚠️
...rces/HtmlTinkerX/HtmlCrawler.StructuredDocument.cs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           v2-speedygonzales     #503      +/-   ##
=====================================================
+ Coverage              58.05%   58.19%   +0.14%     
=====================================================
  Files                    486      488       +2     
  Lines                  36054    36108      +54     
  Branches                7249     7272      +23     
=====================================================
+ Hits                   20930    21013      +83     
+ Misses                 12757    12713      -44     
- Partials                2367     2382      +15     

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