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add deterministic random variables with = syntax

Variables defined with `let x : T = expr` are treated as deterministic
random variables (functions of stochastic parents) rather than
stochastic ones. They can be queried or observed in both the model DSL
and in rejection sampling.

For weighted sampling, only querying deterministic variables is valid.
Observing a deterministic variable suppresses generation of
weighted_sample_iter / WeightedWorld / WeightedSample because there is
no distribution from which to compute a log-likelihood; calling
weighted_sample_iter on such a model produces a clear compile error
that suggests sample_iter as the alternative.

Changes:
- parse: StmtAst gains is_stochastic, supports ~ (stochastic) or =
  (deterministic) separator
- analyze: VariableIR gains is_stochastic propagated from the AST
- codegen: stochastic variables get eval_<v> + evaldist_<v> as before;
  deterministic variables get only eval_<v> (direct expression, cached
  in FeOption); reset_for_weighted pins only stochastic observed vars;
  weighted sampling infrastructure is omitted when any observed variable
  is deterministic
- tests: deterministic_rv.rs covers rejection sampling with a
  deterministic observation and weighted sampling with a deterministic
  query; semantic_err_06 is the negative UI test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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