Tighten C++ ownership filtering and fix operators page headings

[Malcolmnixon]

• C++: track explicitly selected headers in ClangAstParser; a symbol is only owned when its defining header is both under a PublicIncludeRoot and explicitly selected via --api-headers. Prevents transitive-include symbols from leaking into generated docs.
• C++ + .NET: capitalize operators page H1 heading to 'Operators' (proper title-cased heading); rename .NET file to Operators.md (PascalCase convention) with updated navigation links and test keys.
• .NET: fix BuildMethodId to normalize nested-type separators ('/' -> '.') in parameter type FullNames, matching the existing return-type fix.
• Reqstream/verification: update cpp-generator docs to reflect new ownership rules and add transitive-include regression test mapping; fix '#include' rendering typo.

[Copilot]

Pull request overview

This PR tightens C++ API ownership so only symbols defined in explicitly selected --api-headers are documented (preventing transitive-include leakage), and aligns “Operators” page presentation across generators while fixing a .NET XML-doc ID normalization edge case.

Changes:

• C++: Track explicitly selected headers in ClangAstParser and require both PublicIncludeRoot membership and explicit selection for symbol ownership; add regression test and update verification/reqstream docs.
• .NET: Rename the per-type operators page to Operators.md, update links/tests accordingly, and title-case the operators page H1 to Operators.
• .NET: Fix BuildMethodId to normalize nested-type separators (/ → .) in parameter type full names (matching existing return-type behavior).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/ApiMark.DotNet.Tests/DotNetGeneratorTests.cs	Updates expectations/links for Operators.md (PascalCase) in generated output.
test/ApiMark.Cpp.Tests/CppGeneratorTests.cs	Adds regression coverage ensuring transitive-include symbols from non-selected headers are excluded.
src/ApiMark.DotNet/DotNetGenerator.cs	Renames operators page output and normalizes nested-type separators for XML doc method IDs.
src/ApiMark.Cpp/CppGenerator.cs	Title-cases the Operators page H1 headings for class/namespace operator pages.
src/ApiMark.Cpp/CppAst/ClangAstParser.cs	Implements explicit-header tracking and updates ownership logic to require explicit selection.
docs/verification/api-mark-cpp/cpp-generator.md	Updates verification docs to reflect the new ownership/selection behavior and test mapping.
docs/reqstream/api-mark-cpp/cpp-generator.yaml	Updates requirements to reflect header-selection-based ownership and adds the new test mapping.

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