Remove IfElse.jl dependency; use Base.ifelse

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Problem

The strict Downgrade lane (allow_reresolve=false) fails because the resolver was allowed to pick IfElse 0.1.0, whose greedy varargs method

ifelse(args...) = Core.ifelse(args...)

intercepts symbolic Num conditions and forwards to Core.ifelse (which requires a Bool), throwing TypeError: non-boolean (Num) used in boolean context at symbolic source constructors such as Sine (src/Blocks/sources.jl).

Fix

Remove the IfElse.jl dependency entirely and rely on Base.ifelse. On modern Julia, IfElse.ifelse is const ifelse = Base.ifelse, and Symbolics registers a symbolic method on Base.ifelse, so ifelse(sym_cond, a, b) still builds a symbolic ifelse(...) expression rather than eagerly evaluating.

• Replaced all IfElse.ifelse(...) call sites with bare ifelse(...).
• Removed every using IfElse / import IfElse line.
• Removed IfElse from [deps] and [compat].
• src/Blocks/Blocks.jl: the removed line was import IfElse: ifelse, which brought ifelse into module scope so the ifelse(::Bool, ::Parameter, ...) overloads in sources.jl extended it. A bare removal makes those method definitions create a new Blocks.ifelse binding that shadows Base.ifelse for the whole module, so the symbolic Base.ifelse method is no longer visible and Sine/Step/Ramp/… fail to build with MethodError: no method matching ifelse(::Num, …). Replaced with import Base: ifelse so the Parameter overloads extend Base.ifelse and the symbolic method stays in scope.
• Dropped the now-stale Downgrade.yml comment referencing the fntype regression.

Validation (local, Julia 1.12)

• The package precompiles cleanly after the change (both the normal env and the floored downgrade manifest). ModelingToolkitStandardLibrary.Blocks.ifelse === Base.ifelse is true.
• Symbolic tracing confirmed: ifelse(t % T > duty_cycle*T, 1, 0) (Digital Pulse / DigitalPin comparator patterns) build SymbolicUtils.BasicSymbolic ifelse(...) nodes — no eager evaluation.
• Sine (and Square/Step/Ramp/Triangular/ExpSine) build successfully after the import Base: ifelse fix (they fail with a bare removal — confirmed).
• Ran the affected test files (test/analog.jl full pass — exercises Square/Sine/Triangular plus NMOS/PMOS/NPN/PNP transistors and mosfets which use ifelse; test/digital.jl; test/sources.jl; test/isothermal_compressible.jl; test/thermal.jl). No new failures. The pre-existing errors (fntype_X_Y interpolation errors in sources.jl, Valve DomainError in isothermal_compressible.jl, ExtraVariablesSystemException in thermal.jl) reproduce byte-identically on unmodified main and are unrelated to this change (existing MTK 11 / DataInterpolations incompatibilities in the dev depot).
• Strict downgrade harness (julia-downgrade-compat v2.6.1 downgrade.jl) now reports "Successfully resolved minimal versions for . (with extras)". IfElse remains only as a transitive dependency of Static/CPUSummary/etc.; MTKStandardLibrary no longer depends on it directly, so the resolver is no longer exposed to IfElse's ifelse method for the package's own symbolic code.

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