docs: plan call graph resolution pipeline hardening

[forhappy]

Summary

• document the approved architecture for continuation pagination, shared Program IR traversal, performance qualification, real progress, cached enrichment, and compact persistent indexes
• add an implementation-first six-commit delivery plan with current codebase context, stable interfaces, compatibility boundaries, failure behavior, and release gates
• clarify that compass call-graph and compass program call-graph are both first-class surfaces and that Program IR remains open to future semantic enhancements

Why

PR #74 delivered the first interactive call-graph performance hardening and is already merged. This follow-up captures the approved direction for the remaining work before implementation begins, while keeping structural results fast and Program IR enrichment nonblocking.

Impact

Documentation only. No runtime behavior or public contract changes are included in this PR.

Validation

• git diff --check origin/main...HEAD
• verified the branch contains only the call-graph design and implementation plan