Test suite red on master (latest MTK/SymbolicUtils): _clamp import + ExtraVariablesSystemException + fntype_X_Y; blocks downgrade re-enable

[ChrisRackauckas-Claude]

Investigated re-enabling downgrade CI. The suite fails on master with the latest deps (MTK 11.26.8, MTKBase 1.42.2, SciMLBase 3.18.0, DiffEqBase 7.5.5, OrdinaryDiffEq 7.0.0, SymbolicUtils 4.34.3) — so these are pre-existing master failures, not downgrade-floor problems:

1. _clamp import bug (clear): test/Blocks/math.jl:4 does using ModelingToolkitStandardLibrary.Blocks: _clamp, _dead_zone, but _clamp is defined nowhere in src/ (HEAD df33dee — src/Blocks/nonlinear.jl defines only _dead_zone; Limiter uses built-in clamp). → UndefVarError: _clamp.
2. ExtraVariablesSystemException (deeper): SISO Check, Heat flow, Mechanical sensors, AccelerationSensor build unbalanced systems out of mtkcompile under current MTK.
3. fntype_X_Y MethodError: LinearInterpolation/BSplineInterpolation hit MethodError: no method matching fntype_X_Y(::Type{Real}) under SymbolicUtils 4.34.3.

(2) and (3) look like MTK/SymbolicUtils co-evolution breakage and reproduce at newest versions. Fixing (1) alone won't green the suite.

For when master is fixed, the resolve/precompile floors that work on Julia 1.10 (verified): ModelingToolkit ≥11.2.0 (MTK 11.0.0/11.1.0 call Base.ispublic, a Julia-1.11-only function, in @import_mtkbase → UndefVarError: ispublic on 1.10; fixed in v11.2.0), and a DiffEqBase/SciMLBase pairing that avoids both the init kwcall ambiguity (DiffEqBase <6.190.0) and the numargs rrule method-overwrite (DiffEqBase ≥6.210.1 with SciMLBase ≥2.146.0, or keep SciMLBase <2.146.0).

Downgrade CI can't be re-enabled until the master suite is green. (Possibly related to the OrdinaryDiffEq 7.0.0 issues #3721/#3722.)

From the SciML downgrade re-enablement pass.