Investigate and document/consolidate runtime/middlewares and sandbox/middlewares shims

[traoreera]

Context

xcore/kernel/runtime/middlewares/*.py and xcore/kernel/sandbox/middlewares/*.py are entirely one-line re-export shims (from xcore.kernel.middlewares.X import *) pointing at xcore/kernel/middlewares/, which holds the real implementation. Not code duplication in the logic sense, but three directories exist where one canonical implementation lives — confusing for new contributors, and a past changelog entry (2.3.4) already flagged this as a "point d'attention" without resolving it.

Task

1. Grep the codebase (and, if possible, any known external consumers/plugins) for imports of xcore.kernel.runtime.middlewares and xcore.kernel.sandbox.middlewares.
2. If nothing outside the shims themselves needs those two paths: either remove them (with a changelog entry) or add a short docstring in each explaining why the shim exists (e.g. backward compatibility) so it stops looking like accidental duplication.
3. If something does depend on them: document why, and consider a deprecation warning pointing to xcore.kernel.middlewares instead.

Acceptance criteria

• Clear resolution: either the shim directories are removed, or each has an explanatory docstring
• No behavior change for existing imports of xcore.kernel.middlewares (the canonical path)
• make test still green

Files

• xcore/kernel/middlewares/ (canonical)
• xcore/kernel/runtime/middlewares/ (shim)
• xcore/kernel/sandbox/middlewares/ (shim)