Release 5.4.0 so the ForwardDiff extension issquare fix ships

[ChrisRackauckas-Claude]

Please ignore this PR until it has been reviewed by @ChrisRackauckas.

Problem

The newest registered LinearSolve, v5.3.0, is broken against SciMLBase >= 3.40.0. Any solve of a LinearProblem with ForwardDiff.Dual entries throws:

ERROR: UndefVarError: `issquare` not defined in `LinearSolveForwardDiffExt`
Hint: a global variable of this name also exists in SciMLOperators.
Stacktrace:
 [1] solve(::LinearProblem{Nothing, true, Matrix{ForwardDiff.Dual{...}}, ...}, ::Nothing)
   @ LinearSolveForwardDiffExt ~/.julia/packages/LinearSolve/Jvqzc/ext/LinearSolveForwardDiffExt.jl:335

Root cause: LinearSolveForwardDiffExt used issquare unqualified. It resolved only because using SciMLBase inside the extension pulled it in via SciMLBase's @reexport using SciMLOperators. SciMLBase v3.40.0 removed that re-export (it now goes through SciMLOperators as the owner of the operator API), so the name disappeared from the extension's scope.

This is already fixed on master by f1908b9 (#1125), which adds using SciMLOperators: issquare to the extension. The problem is that master still carries version = "5.3.0" — the same version already in the General registry — so the fix is not installable by anyone.

What this PR does

Bumps the version to 5.4.0 so the fix can be registered. Minor rather than patch because master has also gained the TriangularSolve-backed LinearSolveRecursiveFactorizationExt (new weakdep + extension trigger) and the SupernodalLU work since v5.3.0.

Verification

Minimal reproduction (Julia 1.12.4):

using LinearSolve, ForwardDiff
A = [ForwardDiff.Dual(2.0, 1.0) 1.0; 1.0 3.0]
b = [ForwardDiff.Dual(1.0, 0.0), 2.0]
solve(LinearProblem(A, b))

• LinearSolve v5.3.0 (registered) + SciMLBase v3.41.0 -> UndefVarError: issquare not defined in LinearSolveForwardDiffExt
• LinearSolve v5.3.0 (registered) + SciMLBase v3.39.1 pinned -> works (Dual{Nothing}(0.2,-0.12), Dual{Nothing}(0.6,0.04)), confirming SciMLBase 3.40.0 is the trigger
• LinearSolve master (this branch) + SciMLBase v3.41.0 -> works, same values

test/Core/forwarddiff_overloads.jl run against this branch with SciMLBase v3.41.0 passes (exit code 0), including ForwardDiff square-system initialization, DualLinearCache reuse/setu!/partials-validity, sparse, PureKLU and SupernodalLU dual cases.

Downstream impact: this is what makes SciML/PreallocationTools.jl test/core_nesteddual.jl error on unmodified master today (DiffCache Nested Duals), since it solves stiff ODEs under nested ForwardDiff.

Follow-up worth considering

v5.3.0 is broken in place for every user who resolves SciMLBase >= 3.40. A retroactive [SciMLBase] compat cap of 3.39 on LinearSolve 5.3.0 in the General registry would stop new installs from landing on the broken combination.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Yb5kCpT5SRzTrhppKSh1n7

[ChrisRackauckas-Claude]

Downstream confirmation: SciML/PreallocationTools.jl at ac5aeb9 (unmodified master), Julia 1.12.4, GROUP=Core julia --project=. -e "using Pkg; Pkg.test()".

Before (registered LinearSolve v5.3.0 + SciMLBase v3.41.0):

DiffCache Nested Duals: Error During Test
  LoadError: UndefVarError: `issquare` not defined in `LinearSolveForwardDiffExt`
Test Summary:          | Pass  Error  Total     Time
DiffCache Nested Duals |    2      1      3  2m18.7s
ERROR: Package PreallocationTools errored during testing

After (same checkout, Pkg.develop of this branch, SciMLBase v3.41.0), whole Core group green:

Test Summary:       | Pass  Total  Time
Developer Interface |   16     16  4.1s
Test Summary:      | Pass  Broken  Total   Time
DiffCache Dispatch |   82      10     92  16.3s
Test Summary:       | Pass  Total      Time
DiffCache ODE tests |    7      7  14m35.1s
Test Summary:      | Pass  Total  Time
DiffCache Resizing |   64     64  5.9s
Test Summary:          | Pass  Total     Time
DiffCache Nested Duals |    4      4  7m15.7s
Test Summary:              | Pass  Total   Time
DiffCache Sparsity Support |    4      4  20.6s
Test Summary:                           | Pass  Total  Time
DiffCache with SparseConnectivityTracer |   10     10  7.9s
Test Summary:  | Pass  Total     Time
Enzyme Support |   26     26  4m26.6s
Test Summary:   | Pass  Total  Time
LazyBufferCache |    2      2  0.2s
Test Summary:          | Pass  Total   Time
GeneralLazyBufferCache |   11     11  13.0s
Test Summary:                   | Pass  Total  Time
Zero, Copy, and Fill Dispatches |   64     64  3.2s
Test Summary:               | Pass  Total  Time
Allocation Regression Tests |   27     27  3.4s
     Testing PreallocationTools tests passed

(times are inflated; the box was running ~100 concurrent Julia jobs)