ENH: Emit real DeprecationWarnings from mc_compute_stationary / mc_sample_path (or bless them)

Problem

quantecon/__init__.py has carried the comment "Imports that Should be Deprecated with markov package" above mc_compute_stationary and mc_sample_path (markov/core.py:696,711) across many release cycles — but neither function warns. The deprecation exists only for people reading our source: users get no signal, so eventual removal would feel abrupt, while keeping the aliases forever contradicts the stated intent. Both are thin wrappers over MarkovChain.

Proposed change

Decide, then implement — either branch is small:

A. Deprecate for real: add warnings.warn(..., DeprecationWarning, stacklevel=2) naming the replacement (MarkovChain(P).stationary_distributions / MarkovChain(P).simulate(...)) and a target removal version; changelog entry; pytest.warns test. This matches the module docstrings' direction, and given #865 is deciding shim-removal scheduling, the same release train could carry both notices.

B. Bless them: delete the comment, add both to the documented API, add tests.

Acceptance criteria

• Behavior matches an explicit decision recorded in this thread; no "deprecated only in a comment" state remains
• Changelog entry present

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

Start with quantecon/init.py and markov/core.py at mc_compute_stationary and mc_sample_path, then review the module docstrings and issue #865 for the deprecation context. Record an explicit decision between deprecating or blessing the wrappers, add the corresponding tests and changelog entry, and ensure no comment-only deprecation remains.