Fail closed for unqualified bounded group defaults

[AlexanderFengler]

Summary

• fail closed when generic safe-prior generation reaches a unique group-only intercept on a finitely bounded identity scale
• preserve explicit exact/wildcard priors, transformed-link defaults, unbounded parameters, slopes, matched deviations, and calibrated exact/black-box HDDM hierarchies
• give users an actionable choice between a support-respecting link and an explicit centered natural-support hierarchy
• document the release policy in the group-prior guide and HSSM 0.5.0 changelog

Closes #1269.

Why this is the release bridge

The rejected centered hierarchical TruncatedNormal experiment must not become a default. The generic generated group hierarchy is unbounded, while likelihood bounds are not a hard prior-support contract. Until a bounded hierarchy has passed the separate causal and end-to-end qualification, requiring an explicit modeling decision is the smallest safe production behavior.

This PR contains no TruncatedNormal implementation, sampler harness, benchmark artifact, or research dependency.

Scope boundary

• only safe-generated, unique unmatched group-only intercepts are guarded
• only an effective identity link with at least one finite HSSM response bound triggers the error
• exact and wildcard explicit priors are never rewritten or blocked by this guard
• transformed links keep the generated hierarchy on the unconstrained predictor scale
• exact/black-box HDDM likelihoods keep their calibrated natural-support group hierarchies, with the existing warning that exact coefficient bounds are not applied
• group-only slopes and matched group deviations do not inherit response bounds
• prior_settings=None remains available when the user supplies the complete prior policy

Validation

• 1,215 non-slow tests passed
• 465 focused prior/parameterization/graph tests passed with Bambi 0.20.0
• the same 465 focused tests passed at the supported Bambi 0.19.0 floor
• transformed-link and explicit natural-support integration tests build connected graphs with finite initial log density
• Ruff, formatting, Pyrefly, mypy, strict MkDocs, and git diff --check passed

Stack

Temporarily targets #1272 because this guard is defined on top of its population-location ownership policy. Retarget to main after #1272 merges.

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