[WIP] switch SabreDAG's control flow representation from cloned DAGCircuits to Block IDs.

[Janak-Ruia]

Summary

Fixes #15378.
This PR refactors SabreDAG’s control-flow handling to avoid cloning DAGCircuit instances. Control-flow blocks now reference lightweight Block IDs instead of owning full circuit copies.

Details and comments

Changes

InteractionKind::ControlFlow stores Block IDs rather than DAGCircuit clones

from_control_flow links to existing parent blocks instead of creating new circuits

Routing resolves Block IDs from the parent block collection during recursion

Impact

This removes redundant circuit cloning, reduces memory usage, and improves Sabre swap mapping performance—especially for deeply nested control-flow circuits.

AI tool used: Google's Antigravity with Gemini 3

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Janak Ruia seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}