[bgen] Replace XPath lookups with Dictionary in DocumentationManager (#25521)

DocumentationManager.TryGetDocumentation was calling `doc.SelectSingleNode("/doc/members/member[@name='...']")` for every member (~30K calls for tvOS). Each XPath query traverses the entire XML DOM, making this O(n*m) where n=members queried and m=total members.

Replace with a `Dictionary<string, XmlNode>` built once at construction time, giving O(1) lookups thereafter.

Performance (tvOS, wall clock time):
- Before: 270s, peak RSS 363MB
- After: 28s, peak RSS 358MB
- Speedup: 9.6x (242s saved)

Generated code: verified identical (git diff = empty).

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>