Restore dirty LED for tabs with unseen changes (#715)

[Hirogen]

Fixes #715

Problem

The "dirty" LED stopped working in 698d139, when the LED logic moved into LedIndicatorService. The dirty LED is the magenta marker in a tab icon's top-right corner that shows the tab has received new lines you haven't seen yet.

• Nothing set LedState.IsDirty = true. The old rule in LogTabWindow.OnFileSizeChanged was lost in the refactor, so the marker never lit up.
• There were two LedState objects per window. The service created its own LedState in RegisterWindow, while LogTabWindow updated tail, sync and dirty on LogWindowData.LedState. The activity animation (UpdateWindowActivity / OnAnimationTick) drew icons from the service's copy. So whenever lines arrived, the tab icon showed tail as "On" and dropped the time-sync bar, whatever the real state was.
• Activating a tab never cleared dirty. OnDockPanelActiveContentChanged sets CurrentLogWindow first. So OnTabControllerWindowActivated hit its "already current" guard and returned before its clear-dirty code. This was hidden because dirty was never set.

Changes

• ILedIndicatorService.RegisterWindow(window, state) now takes the window's own LedState, so the service and LogTabWindow share one instance.
• UpdateWindowActivity(window, lineDiff, markDirty): LogTabWindow marks a tab dirty when it isn't the current tab or its follow-tail isn't On. This is the pre-refactor rule. The time-sync activity in LogWindowCoordinator passes false.
• New ClearDirty(window) is called from BindActiveLogWindow, which both activation paths reach, and from OnTailFollowed for the active tab.
• The "Don't dirty LED" highlight action still works because FileSizeChanged is suppressed upstream in LogWindow.CheckFilterAndHighlight.

Tests

New tests in LedIndicatorServiceTests, written first and failing before the fix:

• dirty is set on activity when markDirty is true, and not set when it's false
• tail and sync state are kept in the activity icon
• dirty and tail state survive the activity decay to 0
• ClearDirty resets the state and raises the clean icon