🔙 Backport nanobind 3 and drop x86 macOS support

[denialhaag]

Description

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Backports #2209, #2259, and #2286 to v3.x.

This adopts nanobind 3 split mode, raises the minimum supported Python version to 3.11, publishes one cp311-abi3 wheel for GIL-enabled CPython and one cp315-abi3t wheel for free-threaded CPython, and adds nanobind-backend as a runtime dependency.

It also drops x86 macOS from CI and release matrices, keeps debug and release C++ coverage on Apple silicon, and raises the minimum macOS deployment target to 13.3.

The backport includes DD ownership fixes and synchronization for shared DD, IR, and QDMI state. It uses nanobind 3.0.1 because 3.0.0 is yanked, keeps the removed MLIR and QIR stacks absent, and preserves the v3.x FoMaC and neutral-atom interfaces.

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This PR and its contents were created with the assistance of GPT-5.6 Sol via Codex.

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.
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[codecov]

Codecov Report

❌ Patch coverage is 88.28829% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/qdmi/driver/Driver.cpp	88.1%	12 Missing ⚠️
include/mqt-core/ir/Register.hpp	50.0%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[denialhaag]

@burgholzer, to keep them reviewable so that we have a good overview of what goes into 3.10.0, I'm splitting the backports into logical chunks. I hope that makes sense. This one is likely ready, but I want to have a final look before requesting a review tomorrow.

[burgholzer]

Just found two small things, otherwise this looked good while browsing through 👍🏼

And yeah, definitely makes sense to chop up the backports into logical chunks 👍🏼

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