Centralize native runtime, overlay sessions, and diagnostics

[KeyffMS]

Scope

This PR is part of the 0.5.0.50 KISS/DRY/SPoA/SPoT milestone and targets the 0.5.0.50-alpha integration branch.

Changes

• introduce an explicit overlay-session aggregate for primary and transient resources;
• separate magnifier rendering, target availability, and timing policy;
• decompose native menu tracking into signal, enumeration, policy, and publication components;
• confine raw P/Invoke declarations to one native interop boundary;
• route production diagnostics through one structured authority;
• add focused native, timing, diagnostics, and overlay-session tests.

Audit issues

Implements #55, #56, #57, #58 and #64.

Validation

The guarded Windows workflow reconstructs the reviewed patch on the current integration head, then requires:

• clean patch application and git diff --check;
• .NET 8 restore;
• Release build with warnings treated as errors;
• complete MSTest suite with TRX output;
• self-contained Windows x64 publication;
• source publication only after all checks pass.