Switch Tensor to use ArcArray instead of Array

[ihincks]

The motivation for this change in what's stored by a tensor is to allow a cheap cloning mechanism that, in particular, a QuantumProgram can exploit. Since some arrays it deals with may be large, we don't want to force it to clone entire arrays when it needs to do things like fan out a tensor to multiple descendent nodes when evaluating the graph.

See this one commit for the diff being introduced here: 257cd56

PR Stack

• Parent: Refactor ProgramNode::call to flatten+call_flat+unflatten #16298
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• Full Stack

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code: Claude 4.7

[qiskit-bot]

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

• @Qiskit/terra-core

[coveralls]

Coverage Report for CI Build 26910624413

Coverage decreased (-0.006%) to 87.465%

Details

• Coverage decreased (-0.006%) from the base build.
• Patch coverage: 3 uncovered changes across 1 file (68 of 71 lines covered, 95.77%).
• 12 coverage regressions across 6 files.

Uncovered Changes

File	Changed	Covered	%
crates/providers/src/tensor.rs	68	65	95.59%
Total (2 files)	71	68	95.77%

Coverage Regressions

12 previously-covered lines in 6 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/parse.rs	6	97.63%
crates/providers/src/tensor.rs	2	94.8%
crates/circuit/src/parameter/parameter_expression.rs	1	91.03%
crates/circuit/src/parameter/symbol_expr.rs	1	73.88%
crates/qasm2/src/expr.rs	1	93.82%
crates/qasm2/src/lex.rs	1	92.29%

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Coverage Stats

Relevant Lines:	124528
Covered Lines:	108918
Line Coverage:	87.46%
Coverage Strength:	961673.64 hits per line

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💛 - Coveralls