Expose public accessor for CycleAwareCircuit hierarchical Design

[HLEE80]

Context

During PE_INT integration, we found that hierarchical PyCircuit flows may need access to the compiled hierarchical Design object produced by CycleAwareCircuit.

Today this object is stored in the private _v5_design field. Downstream code and CLI/build integration can technically read that field, but doing so creates a dependency on a private implementation detail.

Request

Please expose a stable public accessor for the hierarchical design, for example:

CycleAwareCircuit.hierarchical_design

Expected behavior:

• Returns the compiled hierarchical Design object when hierarchical compilation is enabled and available.
• Returns None or another explicit documented value when no hierarchical design is available.
• Allows CLI/build code and downstream designs to avoid private _v5_design access.

Motivation

This would make hierarchical PyCircuit integration more robust and easier to audit. It also helps downstream designs keep a clean public-API boundary instead of relying on framework internals.

Related cleanup

This came up while removing private API usage from the PE_INT design flow.

[zhoubot]

Implementation review for PR #64:

• Reviewed the public accessor implementation and tests.
• CycleAwareCircuit.hierarchical_design exposes the compiled hierarchical Design object without requiring downstream code to read the private _v5_design field.
• The flat-circuit behavior is explicit and tested: the accessor returns None when no hierarchical design is available.
• The implementation preserves existing emit_mlir() behavior and does not expand the public surface beyond the requested read-only property.

Review verdict: no blocker found. This is the right minimal public API for the integration need described here.