Add rendering metadata and sitemap route controls

[magiccodingman]

What changed

• safely add these declarations to the output copy of the root loader and every canonical snapshot HTML document:

<meta name="rendering-mode" content="static-prerendered">
<meta name="snapshot-protocol" content="1">

• use AngleSharp DOM parsing, duplicate/conflict checks, semantic fingerprint verification, and a safe preserve-with-warning fallback rather than regex insertion
• leave source files untouched and do not annotate case redirects or prefix gateways
• introduce sitemap representation metadata under urn:snapshot-protocol:rendering:1
• keep ordinary sitemap URLs prerenderable by default
• treat <render:representation>client-rendered</render:representation> as a route-level instruction to leave that URL to the SPA
• accept existing static-prerendered and legacy prerendered declarations without duplication
• fail discovery before browser startup when an explicit --route contradicts a sitemap client-rendered declaration
• annotate successfully planned canonical URLs in the ZIP's sitemap copies as static-prerendered
• annotate local sitemap-index child entries only when every URL in that child sitemap is represented by the completed snapshot plan
• preserve gzip sitemap format when annotating .xml.gz

Behavior

Default sitemap URL

<url>
  <loc>https://example.test/docs</loc>
</url>

The route is prerendered as before. The output sitemap copy receives:

<render:representation>static-prerendered</render:representation>

Client-rendered route

<url>
  <loc>https://example.test/account/live</loc>
  <render:representation>client-rendered</render:representation>
</url>

Snapshot skips the route. An explicit override for the same route is an early configuration error rather than silently choosing one instruction.

Safety

• all HTML mutation uses a DOM parser
• existing conflicting or duplicate metadata causes the original output HTML to be preserved with PROCESS103
• serialized HTML is reparsed and compared using a semantic fingerprint that excludes only the two managed metadata elements
• sitemap elements are matched by expanded XML namespace name, not by a hardcoded prefix
• existing recognized sitemap declarations are preserved and never duplicated
• source HTML and sitemap files are never modified

Validation

Added coverage for:

• HTML metadata insertion, idempotence, conflicts, script/text preservation, and validation-only processing
• default, legacy prerendered, static-prerendered, and client-rendered sitemap discovery
• explicit-route conflicts before renderer invocation
• output loader, generated snapshot, and developer-provided canonical HTML annotation
• absence of metadata on redirects
• mixed sitemap indexes and fully-prerendered child summaries
• gzip sitemap round trips
• end-to-end Blazor output metadata

The XML and HTML conventions are documented in docs/rendering-metadata.md.