fix: fix both pause/unpause and step into with lots of tests

[Copilot]

Fixes stale pause/unpause state between UI and Core, and replaces non-monotonic DateTime.UtcNow timeout measurement in test polling helpers with Stopwatch.

Description of Changes

• DisassemblyView.axaml.cs: Track visual-tree attachment state; activate/deactivate view model on DataContextChanged to handle attach-before-DataContext lifecycle ordering.
• StepOverAsmUiTests.cs / StepIntoAsmUiTests.cs: Replace DateTime.UtcNow with Stopwatch in WaitUntil polling helpers for monotonic elapsed-time measurement.
• DisassemblyViewUiTests.cs: Regression test for attach-before-DataContext ordering and pause event propagation; uses runtime null guard instead of null-forgiving operator on reflected method.

Rationale behind Changes

DisassemblyView could be attached to the visual tree before its DataContext is set, leaving DisassemblyViewModel never activated and the disassembly panel unresponsive to pause events. DateTime.UtcNow in test timeout loops is susceptible to NTP/clock adjustments causing spurious timeouts; Stopwatch is monotonic.

Suggested Testing Steps

Run the UI headless tests — they cover the attach-before-DataContext ordering and step-into/step-over pause state transitions. The original race condition is non-deterministic and difficult to reproduce manually.

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