ci: re-enable downgrade CI with corrected [compat] lower bounds

[ChrisRackauckas-Claude]

Re-enables the disabled Downgrade workflow (removes the if: false guard) and replaces the previous, self-contradictory [compat] lower bounds with clean, minimal, verified ones.

What was wrong before

The earlier floors could not resolve on the downgrade run, and the "fixes" left comma-ranges that re-permitted versions below the intended floor (e.g. Compat = "4.18.1, 3.0, 4", SciMLBase = "2.155.1, 2, 3.1", Reexport = "1.2.2, 1.0", DataStructures = "0.19.5, 0.19"). Those lower ranges defeat the bump, and the floors were also raised to the latest patch rather than the lowest working version.

Corrected bounds (lowest that resolve + pass on Julia 1.10)

Compat:           "2.2, 3.0, 4"  -> "4.15"        # NonlinearSolveBase requires Compat >= 4.15
DataStructures:   "0.18, 0.19"   -> "0.19"        # 0.18 too old for the ModelingToolkit/SymbolicUtils stack
FunctionWrappers: "1.0"          -> "1.1.3"       # 1.1.1 fails to precompile ("Module IR does not contain specified entry function"); 1.1.3 fixes it
Reexport:         "0.2, 1.0"     -> "1.2.2"       # BracketingNonlinearSolve requires Reexport >= 1.2.2
SciMLBase:        "1.73, 2, 3.1" -> "2.154, 3.1"  # DiffEqBase 6.217 needs FunctionWrappersWrappers 1, which SciMLBase provides from 2.154

Why the lower ranges had to be dropped: DiffEqBase = "6.217, 7" (its existing floor) already forces SciMLBase >= 2.143 and FunctionWrappersWrappers = 1; SciMLBase 1.73/2.0 can never resolve against that, so the 1.73 and the < 2.154 part of the 2 range are dead. The disjoint upper 3.1 range (DiffEqBase 7 / SciMLBase 3 era) is kept since it is above the floor and genuinely declared.

DiffEqBase, ODEInterface, and SciMLLogging floors are unchanged. julia = "1.6" is kept, and the workflow julia-version stays 1.10.

Verification

Reproduced the downgrade locally on Julia 1.10.11 in an isolated depot by pinning every non-stdlib dep to its compat floor, then iterating to the lowest resolvable + test-passing set. Full Pkg.test() at the pinned floors:

Explicit Imports         | Pass 2/2
Algorithms               | ran (no assertions)
Saving                   | Pass 3/3
Mass Matrix              | Pass 4/4
Jacobian Tests           | Pass 1/1
Callback Tests           | Pass 2/2
Initialization Tests     | Pass 13/13
MTK Initialization Tests | Pass 55/55
Testing ODEInterfaceDiffEq tests passed

Local downgrade suite on Julia 1.10: PASS.

Ignore until reviewed by @ChrisRackauckas.

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