Align root test dependency floors

[ChrisRackauckas-Claude]

Ignore this PR until reviewed by @ChrisRackauckas.

Summary

• add the root test-extra floor NonlinearSolve = "4.20.3" to match OrdinaryDiffEqNonlinearSolve
• add the root test-extra floor DifferentiationInterface = "0.7.18" to match the source-tree Rosenbrock packages

The root project had no compat entries for either test extra. The downgrade resolver therefore selected NonlinearSolve 4.20.2, which became unsatisfiable after the monorepo packages were promoted as path dependencies because OrdinaryDiffEqNonlinearSolve requires 4.20.3 or newer. Adding that floor exposed the same masked problem for DifferentiationInterface 0.7.12 versus the source-tree requirement of 0.7.18.

These are test-environment constraints only; the package's runtime dependency surface and public API are unchanged. The existing downgrade workflow provides regression coverage by resolving the old-style test extras before promoting the source packages.

Validation

• clean-base pinned downgrade reproduction: failed before tests with NonlinearSolve 4.20.2 versus source requirement 4.20.3 - 4
• one-floor diagnostic: selected NonlinearSolve 4.20.3, then failed before tests with DifferentiationInterface 0.7.12 versus source requirement 0.7.18
• final pinned downgrade graph, Julia 1.10.11, GROUP=InterfaceI, allow_reresolve=false: passed (455 pass, 7 pre-existing broken); selected NonlinearSolve 4.20.3 and DifferentiationInterface 0.7.18
• promoted manifest stayed byte-identical through build and test: a72d68117bd460d0ef97c50b4800215fa504625df82eb789a1237d6422b48f31
• current graph, Julia 1.12.6, GROUP=InterfaceI, Pkg.test(): passed (497 pass, 7 pre-existing broken); selected NonlinearSolve 4.21.1 and DifferentiationInterface 0.7.20
• Runic 1.7.0 full-tree check: passed
• Project.toml parse/floor assertions and git diff --check: passed

[ChrisRackauckas-Claude]

Validation scratchpad for commit 7d0d100cc095bd394d7c89c6d5d82de6c9885483 on base abc6fddec8da8ba59308fd59c131df9edeb95985:

• Pinned input: Julia 1.10.11, General ebea3c922b48a58438335288df661a3b4b2f85ae, SciML/DowngradeWorkflow action PR Update CI URLs to point to new caching infrastructure #56 at 28fff4537e765348717821087862dc537353e6a3, and source-promotion logic from SciML/.github PR RK Method for Third Order ODEs  #119 at 4d16630091d8a7c4397f0fe3653d88beaad7b15c.
• Clean base selected NonlinearSolve 4.20.2; locked Pkg.test failed before test execution because promoted OrdinaryDiffEqNonlinearSolve 2.4.0 requires NonlinearSolve 4.20.3 - 4. Reproduction log SHA-256: 33274d2169d34cb6c4f99e0ddbb8eac6f417dfeaeb18f8f4c8fdd76733527b81.
• With only the NonlinearSolve floor, the resolver selected NonlinearSolve 4.20.3; locked Pkg.test then exposed the masked DifferentiationInterface 0.7.12 conflict with promoted OrdinaryDiffEqRosenbrock 2.4.2, which requires 0.7.18 - 0.7. Diagnostic log SHA-256: 2005223963750b3e633a239dbdac77db71da0dd07b833c6472ba15cf3c419443.
• With both floors, the pinned graph selected NonlinearSolve 4.20.3 and DifferentiationInterface 0.7.18; GROUP=InterfaceI passed with 455 passes and 7 pre-existing broken tests. The resolved manifest SHA-256 was a72d68117bd460d0ef97c50b4800215fa504625df82eb789a1237d6422b48f31 before promotion, after promotion, after build, and after locked testing. Green log SHA-256: 87ad63ed1f794ccd67537a9876a7ae5cf663da5033d60cc70b22d4146bd45754.
• An independent current-graph GROUP=InterfaceI Pkg.test() on Julia 1.12.6 selected NonlinearSolve 4.21.1 and DifferentiationInterface 0.7.20; it passed with 497 passes and 7 pre-existing broken tests in 1,613.67 seconds. Log SHA-256: befbd63c0a33de5156a5ec600d12f0306bc0448cb84cc2b3da16192fa6de0a4f.
• Full-tree Runic 1.7.0, TOML floor assertions, and git diff --check all exited successfully.

The pinned wrapper verified that its registry tree did not change during the run and restored both tested worktrees to their pre-run Project/Manifest state.

[ChrisRackauckas-Claude]

Post-timer CI classification for exact head 7d0d100cc095bd394d7c89c6d5d82de6c9885483:

• Downgrade Sublibraries run 29896446982 is terminal at 42 successful / 15 failed jobs.
• The 15 failures preserve the independently reproduced clean-base 7/7/1 split:
  • seven MuladdMacro 0.2.1 / PureKLU 1.1.x conflicts, addressed by draft Align MuladdMacro downgrade floors with PureKLU #3981 with the verified MuladdMacro 0.2.4 boundary;
  • seven recursive path-source reconciliation failures, requiring the shared action/workflow repair tracked by Reconcile path-source constraints in downgrade tests .github#118 and Resolve recursive path-source dependency floors julia-actions/julia-downgrade-compat#56;
  • one DiffEqDevTools hosted runner/log-loss result whose exact clean-base replay with the deployed action and registry resolves successfully with exit 0.
• None of the 15 failures is caused by the two root test-dependency floors merged here. The exact locked/current root validation remains 455 pass / 7 existing broken on Julia 1.10 and 497 pass / 7 existing broken on Julia 1.12, with the locked manifest unchanged.

No test was skipped, silenced, or weakened.