Spell out the SupernodalLU threshold test variable

[ChrisRackauckas-Claude]

Ignore until reviewed by @ChrisRackauckas.

Summary

• rename the local thr loop variable in the SupernodalLU Core test to threshold
• clear the two Typos findings introduced by SupernodalLU: fix block factorization on the direct-BLAS backends #1119 without changing solver behavior or reverting Import issquare from SciMLOperators in ForwardDiff extension #1125/Import cache_operator from SciMLOperators in Core tests #1126

Root cause

A clean upstream/main checkout reproduced exactly two Typos 1.48.0 findings at test/Core/supernodal_lu.jl:176-177. An automated bisect identified 4c58ce272a79969d45eaa640bdf74c2a437263bf (#1119) as the first bad commit.

Verification

• GROUP=Core JULIA_NUM_THREADS=2 julia +1.12 --project=. -e 'using Pkg; Pkg.test()' — passed (2,299 passes; three pre-existing broken cases; SupernodalLU internals 72/72)
• typos-cli 1.48.0 . — passed with zero findings
• julia +1.12 --project=<Runic env> -m Runic --check --diff test/Core/supernodal_lu.jl — passed
• julia +1.12 --project=<Runic env> -m Runic --check --diff . — passed
• git diff --check — passed

[ChrisRackauckas-Claude]

Closing this as an exact duplicate of #1124. The current-main rebase and complete local Core/Typos/Runic evidence have been consolidated into #1124; this PR had no unique code changes.