✨ Preserve reusable gates across OpenQASM import and export

[burgholzer]

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Description

Preserve OpenQASM gate definitions as reusable QC functions and emit supported helpers as dependency-ordered OpenQASM gate declarations. Straight-line gates use the unitary-function contract; structured gate bodies remain private generic functions.

This PR is stacked on #2340. Native MLIR call-graph traversal replaces recursive ordering and preserves the 64-level frontend limit. Gate-expression export keeps integer-to-float promotion, so expressions such as a converted loop index divided by two retain floating-point semantics.

The exporter retains the current classical-register safeguards. Gate bodies cannot use mutable classical state; generic OpenQASM subroutines and a classical-reference call ABI are outside this change.

Codex implemented and tested the changes with independent OpenQASM/MLIR specialist review. Regressions cover a 50-helper dependency graph, numeric loop expressions, strict reparsing, and QC ↔ QCO round trips.

Integrated-stack validation: release build; 3,857 registered CTest cases with one expected skip; 259 Qiskit translation tests; whole-file C++ lint and repository lint. Stub generation passed without changes. Hosted CI remains separate and has not been claimed as passed.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[codecov]

Codecov Report

❌ Patch coverage is 91.70507% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../Dialect/QC/Translation/TranslateQCToOpenQASM3.cpp	92.1%	14 Missing ⚠️
...lib/Dialect/QC/Translation/OpenQASMToQCEmitter.cpp	89.4%	4 Missing ⚠️

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