Add tracking issue doc for tail-call stepping in ReadyToRun assemblies

[Copilot]

Description

Creates a tracking issue document for the tail-call stepping correctness bug identified in PR #123640 discussion.

Problem

IsTailCall() in controller.cpp assumes it can immediately resolve the destination MethodDesc from a call IP. This breaks for cross-R2R tail calls through ExternalMethodFixup stubs, which use deferred callback resolution via TraceManager().

Changes

• Added docs/design/coreclr/tailcall-stepping-r2r-tracking-issue.md documenting:
  • Root cause: TRACE_EXTERNAL_METHOD_FIXUP sets address = NULL by design
  • Affected code paths in controller.cpp and stubmgr.cpp
  • Potential solution approaches
  • References to related PRs (Update stepping through MulticastDelegate under the Debugger #108414, Update stepping through ExternalMethodFixup under the Debugger #108942, Fix AV in IsTailCall by checking for NULL trace address #123640)

Note

Document serves as issue body content for manual GitHub issue creation, as direct issue creation is not available from this environment.

Original prompt

Create a tracking issue as described in the following comment and PR discussion: #123640 (comment)

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