Add ReactivePage.InvalidateLayout() for runtime layout rebuilds

[Aaronontheweb]

Problem

ReactivePage<TViewModel>.OnNavigatedTo builds and caches _layoutRoot exactly once and never re-runs BuildLayout(). Any value captured inside BuildLayout() and baked into a layout-node field (like HeightAuto's SizeConstraint.Auto record) is permanently frozen at first-navigation time. Consumers have no way to trigger a rebuild — terminal resize handlers can't force the layout to re-evaluate height/width-dependent constraints.

Fix

Add protected void InvalidateLayout() to ReactivePage<TViewModel>. Discards the cached _layoutRoot and rebuilds via BuildLayout(), preserving everything the user cares about (Subscriptions, KeyBindings, and user focus when the target node survives the rebuild).

Refactor: move the framework's IInvalidatingNode.Invalidated → RequestRedraw subscription to a dedicated _layoutSubscriptions CompositeDisposable so InvalidateLayout can tear it down without touching user code.

Consumer usage

// In OnBound, post #219:
ViewModel.Input.OfType<IInputEvent, ResizeEvent>()
    .Subscribe(_ => InvalidateLayout())
    .DisposeWith(Subscriptions);

Hardened lifecycle contract (post review)

Initial commit was functional but fragile. Code review surfaced lifecycle / exception-safety / UX gaps that this PR now addresses:

Lifecycle state (_isActivated / _isDisposed / _isRebuilding flags):

• Throws ObjectDisposedException if InvalidateLayout or OnNavigatedTo is called after Dispose. Verified R3 CompositeDisposable.Clear/Add silently no-op on a disposed bag, so without the guard a late InvalidateLayout would build a fresh tree, OnActivate every node (starting timers/animations), then leak the whole tree.
• Throws InvalidOperationException on re-entrant InvalidateLayout (from inside BuildLayout or a node lifecycle callback) — prevents unbounded recursion and divergent state.
• No-ops when inactive (before first OnNavigatedTo, or between OnNavigatingFrom and the next OnNavigatedTo) — prevents resurrecting an inactive page and the resulting double-subscribe + double-activate on the next navigation.
• Idempotent OnNavigatedTo — skips wiring path when already active so a user override calling base.OnNavigatedTo() twice doesn't pile up duplicate _layoutSubscriptions.
• Idempotent Dispose.

Exception-safe build-then-swap:

• Builds the new tree FIRST, then OnActivate's it, then atomically swaps _layoutRoot, then deactivates the old. If BuildLayout throws, the page keeps its existing layout — no half-state with cleared subscriptions and a null root.
• BuildLayout returning null throws InvalidOperationException with a clear message instead of silently leaving _layoutRoot null and rendering blank.

Focus preservation:

• No longer unconditionally re-applies FocusPolicy. If the user has Tab'd to a focusable AND that node still exists in the new tree (common case when BuildLayout reuses page-field nodes), focus is preserved. Only when the focused node is orphaned does the policy default re-apply.

Redraw trigger:

• InvalidateLayout explicitly calls ViewModel.RequestRedraw() at the end so a resize handler that only invalidates produces a visible change.

What's documented (not enforced)

• Orphan-container leak: ContainerNode.Dispose() recurses into children, so we can't Dispose the old tree without destroying user-held leaves (e.g., a StreamingTextNode page field). The old container's _childInvalidationSubscriptions to those leaves stay live until page Dispose. Repeated invalidate accumulates dead containers. Documented in the XML remarks; consumers using heavy reusable nodes should prefer reactive bindings inside BuildLayout over frequent invalidate.
• User Subscriptions targeting BuildLayout-created nodes: those subscriptions keep firing into orphaned nodes after invalidate. Documented; consumers should bind to page-field nodes (initialized in OnBound and reused) or re-subscribe at the start of each BuildLayout.
• Non-LayoutNode ILayoutNode wrappers: not deactivated by the pattern match (pre-existing behavior in OnNavigatingFrom/Dispose). Documented in remarks.
• Thread affinity: InvalidateLayout must be called on the same dispatcher thread that drives navigation/rendering.

Tests

tests/Termina.Tests/Reactive/ReactivePageInvalidateLayoutTests.cs — 17 tests total (5 original + 12 new):

Test	Covers
TriggersBuildLayoutAgain, ReplacesLayoutRoot	Basic rebuild
PreservesUserSubscriptions, PreservesKeyBindings	User state preserved
SeesUpdatedExternalStateInBuildLayout	Consumer-shaped use case
BeforeOnNavigatedTo_IsNoOp	Inactive-before-first-nav guard
AfterOnNavigatingFrom_IsNoOp	Inactive-between-navs guard
AfterDispose_Throws, OnNavigatedTo_AfterDispose_Throws	Disposed guard
Dispose_IsIdempotent	Idempotent dispose
ReEntrant_Throws	Re-entrancy guard (page re-enters from BuildLayout)
NullBuildLayout_Throws	Null contract enforcement
BuildLayoutThrows_LeavesLayoutIntact	Build-then-swap rollback
RequestsRedraw	Explicit redraw trigger
OldInvalidatingNode_DoesNotDriveRedraws	Old subscription disposed
NewInvalidatingNode_DrivesRedraws	New subscription wired
RepeatedCalls_DoNotAccumulateSubscriptions	No subscription pile-up

The last three close the gap the original tests had — all originals used TextNode which is NOT IInvalidatingNode, so the entire subscription-leak surface was untested.

Test plan

• dotnet test tests/Termina.Tests — 1116/1116 pass (17 InvalidateLayout + 1099 existing).
• No behavior change for existing pages that don't call InvalidateLayout or override the lifecycle methods.
• No new public surface beyond a single protected method (InvalidateLayout).

[Aaronontheweb]

Two concrete issues worth fixing before merge:\n\n1. abandons the old layout root without disposing it, but wrapper nodes such as and only detach child invalidation subscriptions in . If a page reuses a child node instance across rebuilds, those old wrapper nodes can stay alive through the child -> parent invalidation subscription chain instead of being garbage-collected.\n\n2. rebuilds the tree without reconciling focus. On pages using manual focus, can keep pointing at a control from the discarded tree, so subsequent input is routed to a stale node instead of the rebuilt layout.\n\nPlease add a regression test for the focus case as part of the fix.

[Aaronontheweb]

Two concrete issues worth fixing before merge:

1. InvalidateLayout() abandons the old layout root without disposing it, but wrapper nodes such as PanelNode and ContainerNode only detach child invalidation subscriptions in Dispose(). If a page reuses a child node instance across rebuilds, those old wrapper nodes can stay alive through the child -> parent invalidation subscription chain instead of being garbage-collected.

2. InvalidateLayout() rebuilds the tree without reconciling focus. On pages using manual focus, IFocusManager.CurrentFocus can keep pointing at a control from the discarded tree, so subsequent input is routed to a stale node instead of the rebuilt layout.

Please add a regression test for the focus case as part of the fix.