fix: guard a leaf whose parameters are vectors, not scalars

[seabbs-bot]

A leaf with vector-valued parameters silently emitted a Vector into a scalar parameter row. The table walk defines one row per scalar parameter, so there is no correct row for a probability vector.

The guard existed only in Uncertain's constructor, so a fixed leaf reached the plain walk unchecked. Categorical is the motivating case.

What changes

Six lines in _walk_rows! (src/composers/introspection.jl) that throw when a leaf's params are not all Real, naming the leaf and pointing the caller at direct parameter access.

Composing and scoring such a leaf is unaffected and stays supported — only the table view is undefined for it. The test pins both halves: logpdf still works, the table throws.

Provenance, and one half deliberately dropped

Salvaged from a branch an earlier session left uncommitted. That branch also changed leaf_mean/leaf_var to stop reading through free_leaf, so a Truncated leaf stops reporting its untruncated moment. That is a real defect, but #365 fixes it better: it falls back to the free-leaf approximation where Distributions.jl has no closed form, whereas the unconditional version throws a MethodError for Truncated{Gamma}. That half is dropped here in favour of #365, so the two PRs do not collide.

Gates

Targeted verification only, run against this branch: the guard throws for a Categorical leaf, logpdf on the same tree still returns a Real, and an ordinary two-leaf tree still produces its four parameter rows unchanged. The full suite was not run — the machine is saturated with the other agents in this batch, and CI will run it here anyway.

Rebase note

Touches the table walk that #343 rewrites, and uses params_table, which #343 removes in favour of composed_to_table. Whichever merges second needs a small rename in the guard's test. Kept separate from #365 for exactly this reason.

This was opened by a bot. Please ping @seabbs for any questions.

[github-actions]

Try this Pull Request!

Option 1: Julia Package Manager

Open Julia and type:

import Pkg
Pkg.activate(temp=true)
Pkg.add(url="https://github.com/EpiAware/ComposedDistributions.jl", rev="fix/vector-param-guard-and-truncated-moments")
using ComposedDistributions

Option 2: Local Checkout

If you have the repo locally:

git checkout fix/vector-param-guard-and-truncated-moments
julia --project=. -e "using Pkg; Pkg.instantiate()"

[seabbs-bot]

Superseded by #381, which folds this and seven other PRs into one branch off main so the 0.2.0 contract reviews as a single diff. Its gates: test-fast 2379/2379, test-quality 255/255, full docs build clean.

This branch is preserved and the PR body is kept for its rationale — the detail here is the record of why, which #381 summarises rather than repeats. Closing so it does not compete for review.

This comment was posted by a bot. Please ping @seabbs for any questions.