Avoid cache-hit workspace allocations on Julia 1.10

[ChrisRackauckas-Claude]

Stores the large exponential! workspace in a private concrete _ExponentialWorkspace{W} cell, rather than returning it by value from the cache. This removes the 64-byte Julia 1.10 allocation on warmed get_expcache! hits while using only ExponentialUtilities-owned APIs.

The old return-by-value path regressed at commit 3d1e43f; released e044eeb fails the Core allocation assertion on Julia LTS hosted platforms and in the downgrade job. The initial draft used non-public Base.RefValue; this update replaces it with the local cell so ExplicitImports passes.

Validation:

• Julia 1.10 direct cache controls: concrete local cell, cache identity reuse, and zero warmed-hit allocations.
• Julia 1.10 non-Hermitian Complex ExpvCache control: workspace reuse confirmed.
• Julia 1.10 GROUP=Core Pkg.test(; coverage=true): 738 passed, 1 expected broken, 0 failures/errors.
• Julia 1.10 GROUP=QA Pkg.test(; coverage=true): 19 passed, 2 expected broken, 0 failures/errors; ExplicitImports passes.
• Runic check and git diff --check pass.

Draft — please ignore until reviewed by @ChrisRackauckas.