Harden session recovery, input handling, and persistence

[devmobasa]

Preserve drawings through session switches, output changes, failed loads, and shutdown. Tighten session-file validation and corruption recovery, including oversized exit rescue and remembered sessions whose primary file disappears.

Refine pointer/tablet ownership, route clipboard tests through production paste logic, isolate settings fixtures, and split large modules for readability.

Add optional save-generation metadata and bounded probes while preserving legacy load precedence and artifact cleanup rules. Update the session architecture documentation and cover historical session files with compatibility fixtures.

Validation:

• Full local CI: 5,398 core tests with all features and 5,193 with no default features; 514 and 513 configurator tests respectively. Repository, packaging, formatting, and clippy checks pass.
• GTK gate: 88 tests pass on a private headless compositor, with all four required EXECUTED markers.
• The remembered-session regression exercises the real corruption restoration path, verifies home fallback and preserved data, and fails when the old early return is restored.
• Exit rescue covers fitting and oversized drawings through both the worker and joined direct fallback, reopening the saved result through the named-session loader.

Live desktop and physical tablet behavior were not exercised.