Add chronological shell navigation history model

[Hawkynt]

What changed

• Adds a small desktop-shell session-history model with browser-style Back/Forward semantics.
• Stores full ProcessKey values in process locations so PID reuse can never redirect a replayed history entry to a different process.
• Separates Push (new visit) from Replace (state change within the current visit), allowing process-row selection to be remembered without turning every arrow-key move into another Back step.
• Adds a structural breadcrumb model kept deliberately separate from visited history.
• Adds focused nUnit coverage for chronological traversal, forward-branch truncation, duplicate suppression, replace-state behavior, reused-PID identity, and breadcrumb shape.

Design

Behavior was derived clean-room from public platform documentation: browser session history supplies the chronological Back/Forward model, while breadcrumb guidance supplies the separate structural-path model. No external implementation code was copied or adapted, and no dependencies were added.

This is the first coherent slice of the navigation feature. The follow-up commits on this draft PR will wire the model through the existing ShellNavigator/MainWindow seam, add Back/Forward and breadcrumb commands, restore an exact process selection on replay, and extend capture regression coverage.

Validation

The branch was created from current main commit 134a44645910b664521d9b53a5c3098eaf0f65b7. CI is intentionally left to the PR workflow because this environment does not currently provide a usable local .NET SDK.