feat!: total graph evaluation with Result-returning error handling (spec 06)

[minikin]

Summary

• ComputationNode::evaluate, evaluate_arithmetic, and evaluate_bool now return Result<_, UncertainError> instead of panicking on unsupported node/domain combinations. New error variant UncertainError::UnsupportedNode { expected, found }.
• evaluate_arithmetic absorbs the former evaluate_conditional_with_arithmetic (now deleted) and is the total dispatcher for the arithmetic domain, including Conditional nodes.
• evaluate_fresh keeps its infallible -> T signature (used by ~20 existing callers, mostly tests) via an internal .expect() justified by the invariant that graphs built through the public Uncertain<T> API are always well-formed.
• GraphProfiler::get_stats's two now-unreachable .expect() calls replaced with direct indexing.
• All #[should_panic] evaluation tests replaced with Result/error-variant assertions; added tests closing coverage gaps the rewrite exposed.
• See specs/06-total-graph-evaluation.md for the full spec and an "Implementation notes" section documenting why this keeps three separate domain-scoped dispatchers (evaluate/evaluate_arithmetic/evaluate_bool) rather than one universal dispatcher — each is total over a different variant set dictated by T's trait bounds.
• crap_baseline.json regenerated: the three rewritten methods show a higher CRAP score (CC +1 to +5) from the added Result plumbing, but all reached 100% line coverage in this change, so the increase is pure complexity, not a coverage gap.
• This branch was cut before specs 05/18 merged; rebased cleanly onto current main (conflicts only in specs/README.md/MIGRATION_GUIDE.md/CHANGELOG.md, resolved by combining both specs' notes).

Test plan

• just dev green (fmt, clippy -D warnings, tests default + parallel, doc tests, audit, deny, crap regression gate) — reconfirmed after rebase
• Full test suite + doctests pass with --all-features and --features parallel
• cargo bench sanity check on computation_graphs — all three benchmarks within noise
• CHANGELOG.md/MIGRATION_GUIDE.md/specs/README.md updated

https://claude.ai/code/session_01Xw68NoMZnNRLkxjK6Razu7