Fix stale undefined exports (Variable, Unknown)

[ChrisRackauckas-Claude]

Problem

names(Symbolics) currently lists two names that do not resolve in the module, so Aqua.test_undefined_exports(Symbolics) fails and a blanket @reexport using Symbolics leaks an undefined binding into downstream packages (this is what forced the workaround in SciML/Corleone.jl#86).

On Julia 1.12 with Symbolics v7.28.1 / current master:

julia> using Symbolics
julia> filter(n -> !isdefined(Symbolics, n), names(Symbolics))
2-element Vector{Symbol}:
 :Unknown
 :Variable

julia> using Aqua
julia> Aqua.test_undefined_exports(Symbolics)   # FAILS
# Evaluated: isempty([Symbolics.Unknown, Symbolics.Variable])

Root cause

• Variable is exported at src/Symbolics.jl:170, but the struct Variable{T} end (a deprecated shim that redirected to variable(name, idx...; T=T)) was removed in 55a3d1a ("refactor: use @syms parsing for @variables") without dropping it from the export list. The binding has not existed since.
• Unknown was marked @public in 5d78c7a ("refactor: mark Unknown as public"), but SymbolicUtils.Unknown was never brought into the Symbolics namespace, so the public name had nothing to resolve to. SymbolicUtils does not export Unknown, so @reexport using SymbolicUtils does not bring it in either.

Fix

• Drop Variable from the export (the public replacement is variable(...)).
• Import Unknown alongside the other SymbolicUtils names so the @public Unknown resolves (Symbolics.Unknown === SymbolicUtils.Unknown). This matches how Unknown is already referenced throughout the source as SymbolicUtils.Unknown.

Verification (Julia 1.12)

• Before: Aqua.test_undefined_exports(Symbolics) fails ([Symbolics.Unknown, Symbolics.Variable]).
• After: Aqua.test_undefined_exports(Symbolics) passes; filter(n -> !isdefined(Symbolics, n), names(Symbolics)) is empty; Symbolics.Unknown === SymbolicUtils.Unknown; core smoke test (@variables, derivative, variable(:z), Unknown(2)) still works.

Runic: the two changed lines are Runic-clean (sciml style). The file has unrelated pre-existing Runic drift in a precompile block near the bottom, which is left untouched to keep the diff minimal.

---

Please ignore until reviewed by @ChrisRackauckas.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.75%. Comparing base (c736678) to head (bed0571).
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Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1906       +/-   ##
===========================================
+ Coverage   26.12%   80.75%   +54.63%     
===========================================
  Files          56       59        +3     
  Lines        5356     5732      +376     
===========================================
+ Hits         1399     4629     +3230     
+ Misses       3957     1103     -2854     

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