Method ambiguities present; Aqua ambiguity/piracy checks disabled in CI

[docxology]

Summary

Running Aqua with ambiguities=true reports 27 method ambiguities in GraphPPL. The CI
gate (test/runtests.jl:4) disables both ambiguity and piracy checks:

Aqua.test_all(GraphPPL; ambiguities = false, piracies = false, deps_compat = (; check_extras = false, check_weakdeps = true))

Several ambiguities are self-inflicted and can dispatch to the wrong method (or MethodError)
on valid inputs:

• checked_getindex(::NodeLabel, ::FunctionalIndex) vs checked_getindex(nodelabel::NodeLabel, index) — src/graph_engine.jl:376 vs :384
• recursive_size(::Val{1}, ::Vector{T}) vs recursive_size(::Val{N}, ::Vector{<:Vector}) — src/resizable_array.jl:59 vs :63
• <(::VariableRef, ::VariableRef) / <= — src/graph_engine.jl:935-938
• getindex(::NodeCreationOptions, ::NTuple{N,Symbol}) vs getindex(::NodeCreationOptions{Nothing}, ...) — :658 vs :664
• the four getindex(collection::AbstractArray, range::FunctionalRange{...}) overloads — :108,:111,:112

Why it matters

Julia resolves ambiguous dispatch (possibly nondeterministically) or throws MethodError.
For a library that must build the same factor graph deterministically, an ambiguous
size/recursive_size/getindex is a reproducibility risk. Also, the intentional
"cannot compare VariableRef" error path (equalish-<) can instead dispatch into one of the
mutually-ambiguous generic fallbacks.

Suggested fix

1. Re-enable ambiguities = true (and ideally piracies = true) in test/runtests.jl.
2. Add explicit disambiguating methods for the self-inflicted cases (exact signatures listed
   in findings/medium-04-...md).