Add deferred real-world parse probes

[kristjanvalur]

Summary

• add deferred real-world parse/demangle probes collected during branch splitting

Motivation and Context

These probes are intentionally separated because they rely on the combined behavior introduced by all three C++20 split PRs:

• PR Render requires clauses with level-aware template params #314: level-aware template parameter resolution + requires-clause rendering
• PR Support extended C++20 template-param qualifier forms #315: extended template-parameter qualifier forms
• PR Normalize local-source template handling in nested names #316: local-source template handling and nested-name normalization

In other words, this test slice is expected to be meaningful only once the combined behavior from #314 + #315 + #316 is present.

Scope

• test/probe additions only
• no parser or demangler logic changes