MAINT: Add contract tests for the deprecation-shim layer (currently 0% covered)

Problem

All 21 base-level deprecation shims (quantecon/arma.py … robustlq.py) sit at 0% test coverage — nothing asserts that (a) each emits DeprecationWarning on attribute access and (b) every name in each shim's __all__ actually resolves at the top level, which is what the warning text promises. The predictable failure already happened: six advertised names are missing from quantecon (#853 — ckron, gridmake, annotate_nodes, num_compositions_jit, simulate_linear_model, solve_discrete_riccati_system; independently re-verified during this audit).

This issue is the guard-rail, deliberately scoped apart from the fixes: #853 restores the six exports; #865 decides removal timing. Whichever way #865 concludes, the layer should be honest while it exists.

Proposed change

One parametrized test module (~40 lines): for each shim, import it, assert every __all__ name exists on quantecon, assert getattr emits DeprecationWarning, and assert an unknown attribute raises AttributeError. It lands red on the #853 six until that fix merges — coordinate ordering, or xfail those entries with a link.

Acceptance criteria

• Shim modules no longer at 0%; the #853 class of regression cannot recur silently
• The test module is trivially deletable in the same PR that deletes the shims

If #865 concludes with immediate removal, close this as wontfix with a link.

From the July 2026 technical-debt audit (AI-assisted; claims verified against 28d4b3b on 2026-07-25).

Start by inspecting the 21 base-level shim modules under quantecon/, from arma.py through robustlq.py, and compare each all with the top-level exports. Add the proposed parametrized test module for warning behavior, advertised names, and unknown attributes. Check #853 and #865 before finalizing, since the tests may depend on the export fix or become unnecessary if the shims are removed.