feat: add feedback banner and an issue link on fit errors

[jrosskopf]

Inference quality depends on real data shapes we cannot reproduce from synthetic draws, so the users who get a bad fit are the only ones who can tell us what their model and data looked like.

Part of a fleet-wide rollout using the shared DataZooDE/duckdb-extension-banner submodule.

The bug this PR nearly shipped with

I first guarded BayesFitBind and BayesFitInitGlobal. It compiled, the suite passed, and the footer appeared on nothing at all — because the fit runs in the finalize phase (RunFit is called from BayesFitFinalize), so no real failure ever passed through bind or init.

Only running an actually-failing fit against the built shell caught it. The guard now sits on in_out_function and in_out_function_final:

Invalid Input Error: anofox_bayes_fit: unknown model family 'not_a_family'
  (catalog: hier_negbin, censored_aft, pooled_gaussian, payment_delay, payer_alive,
   hier_elasticity, conjugate_anomaly, varying_variance_gaussian)
-> Unexpected? Please report it: https://github.com/DataZooDE/anofox-bayes/issues

feedback.test asserts this path specifically, with a comment recording why bind/init is not enough.

BayesError is deliberately untouched

crates/anofox-bayes-core/src/errors.rs states that the primary caller is an agent and that variants carry machine-readable repair information — the catalog list above exists precisely so a caller can fix its own request. Appending prose to that Display would corrupt the contract.

So the link is added at the C++ boundary, where a human reads the message, and the Rust crate is not modified. Every error a person sees carries the link; every error an agent parses is byte-identical to before. The reasoning is written into anofox_bayes_banner.hpp so it survives this PR.

Verified

• Banner once interactively, silent on repeat, silent when piped
• Footer present with the core's diagnostic intact
• 681 assertions in 17 cases pass, plus the new feedback.test (5 assertions)

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