⚡ Avoid repeated QDMI result and metadata work

[burgholzer]

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Description

Concurrent DDSIM probability and statevector readers now share the existing initialization guard. Dense size queries compute the checked byte count from the DD root, without constructing an exponential vector; undersized buffers are rejected before allocation.

PennyLane decodes validated shots through NumPy and uses the known Python operation name to access its per-converter contract cache. Qiskit reads duration units and scale lazily once per target snapshot, preserving validation and floating-point evaluation order.

Regressions cover cold concurrent readers, dense-size overflow, malformed shots and wire order, repeated operation-name queries, and duration metadata reuse across placements and snapshots. The retained audit records the measurements and limits. Open #2373 touches the same DDSIM source; its multi-program changes remain separate.

Validation: 67 DDSIM tests, 241 client tests, and 102 focused Python frontend tests passed. Full Nox lint and C++ lint passed, with all lines of the three changed C++ files checked. The local 22-qubit probe confirms zero dense allocations for size-only requests and sub-microsecond queries. No hosted CI, Windows, ThreadSanitizer, or live cloud validation is claimed.

These changes affect unreleased v4 functionality, so no standalone changelog or migration entry is added. Codex implemented and checked the changes under explicit authorization; human review remains required before merge.

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 93.33333% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/qdmi/devices/dd/Device.cpp	90.9%	2 Missing ⚠️
python/mqt/core/plugins/pennylane/device.py	85.7%	1 Missing ⚠️

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