fix: register each anonymous variable under a unique context key

[bvdmitri]

Closes #311.

Problem

Every anonymous variable was registered in its Context under the constant key
VariableNameAnonymous = :anonymous_var_graphppl. Context's setindex! routes that to
set!(c.individual_variables, key, val), which is insert-or-update, so when a context creates two or
more anonymous variables (e.g. the x + 1 and y + 1 sub-terms of z ~ Normal(x + 1, y + 1)) each
one silently overwrote the previous. The registry kept only the last.

The factor graph itself was always correct — each anonymous variable is a distinct vertex with the
right edges — so this was latent. But anything enumerating individual_variables(context) (notably
VarDict / getvardict) silently missed the earlier ones.

Change

Added add_anonymous_node!, mirroring the existing add_constant_node!, which registers under
to_symbol(VariableNameAnonymous, label.global_counter) and returns the label explicitly. The four
call sites in materialize_anonymous_variable! now use it.

Constants already work exactly this way (constvar_10, constvar_13, ...), which is what made the
anonymous keying look unintended in the first place.

Compatibility

The node property name is unchanged, so nothing that identifies anonymous variables breaks:

• is_anonymous (graph_engine.jl:737) reads VariableNodeProperties.name
• as_variable(VariableNameAnonymous) filtering compares getname(getproperties(...)), not the key

Both are property-based, so all 14 existing assertions in test/graph_construction_tests.jl pass
untouched. Nothing in src/ or ext/ looks up the literal :anonymous_var_graphppl key.

The one visible change: VarDict / getvardict and Context's show now list one entry per
anonymous variable instead of one in total, and haskey(context, :anonymous_var_graphppl) is now
false. This is the intended fix — those entries were being lost — and it matches how constvar_N
already appears there. Worth knowing for downstream consumers that iterate VarDict.

Tests

New test item in test/graph_construction_tests.jl: a submodel creating two anonymous variables in one
context, asserting the number of distinct anonymous keys in individual_variables equals the number of
anonymous vertices in the graph, and that is_anonymous still identifies both.

Verified it fails on main (2 failures) and passes here. Full suite on Julia 1.12:
259/259 test items, all passing.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.73%. Comparing base (4cc7c6c) to head (b7c46d8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #325      +/-   ##
==========================================
+ Coverage   90.72%   90.73%   +0.01%     
==========================================
  Files          16       16              
  Lines        2263     2267       +4     
==========================================
+ Hits         2053     2057       +4     
  Misses        210      210              

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