Same-shape fast path for restructure(x::Array, y::Array)

[ChrisRackauckas-Claude]

What

Add a same-shape fast path to restructure(x::Array, y::Array): when y already has x's shape, return y directly instead of reshaping.

Why

restructure(x::Array, y) is reshape(convert(Array, y), size(x)...). reshape has no same-shape short-circuit (unlike vec, which returns a Vector unchanged), so even reshaping an array to its own exact shape mints a fresh Array header — reshape(y, size(y)) !== y, 32 bytes on Julia 1.12's Memory-backed arrays. The data buffer is shared (no copy), but the wrapper object is a per-call heap allocation.

That per-call allocation shows up in hot loops. In NonlinearSolve's Newton descent, the linear solve already writes its result into the cached δu buffer and then restructures it back to the state shape — for a plain Vector state that's a no-op that allocates ~once per iteration. On an OrdinaryDiffEq NonlinearSolveAlg (Van der Pol μ=1e5, TRBDF2) that single call was 68% of the solver's per-step allocations; this fast path cuts the solve's total allocation by ~56% with identical results, and it helps every caller that restructures already-correctly-shaped arrays (RecursiveArrayTools, DiffEq, NonlinearSolve, …).

Semantics

Unchanged. The x::Array path already returned a data-sharing reshape, so returning y (which shares its own data) preserves the existing aliasing contract; eltype/shape of the result are identical. A genuine shape change (size(x) != size(y), e.g. vector → matrix) still reshapes exactly as before. Type-stable: for differing ndims the size comparison is a compile-time false, so the reshape branch is always taken.

Patch bump (7.28.0 → 7.28.1); test added to the restructure testset.

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Draft — please ignore until reviewed by @ChrisRackauckas.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.75%. Comparing base (80e3539) to head (bba033c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #497      +/-   ##
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+ Coverage   60.56%   60.75%   +0.19%     
==========================================
  Files          15       15              
  Lines         606      609       +3     
==========================================
+ Hits          367      370       +3     
  Misses        239      239              

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