feat(focus): recover focus when the active element is removed

[chiefcll]

What

Adds automatic focus-loss recovery plus dev-mode diagnostics for the most common focus failure mode in TV apps: the focused element unmounts and the remote goes dead.

Why

Nothing in the framework noticed when the focused element left the tree. removeChild and the delete-flush phase never checked whether the removed subtree contained activeElement(), so when a <Show> toggled, a list item was deleted, or a route swapped while its content was focused, the activeElement signal kept pointing at a destroyed node and key events walked a stale focus path. Every app had to defend against this manually.

How

• Recovery (elementNode.ts): the unified post-mutation scheduler gets a phase 4 — after any pass that actually destroyed nodes (the only way the active element can leave the tree, so ordinary layout/focus passes pay nothing), if no focus change is pending, recoverActiveElement() runs.
• recoverActiveElement() (focusManager.ts): if activeElement() is detached, walk the old focus path to the nearest still-attached, rendered ancestor and call its setFocus() — that ancestor's forwardFocus/selected logic then picks a sensible child. Detachment is verified by checking each node is present in its parent's children array, since removeChild doesn't clear node.parent.
• Opt-out (config.ts): Config.focusLossRecovery, default true.
• Dev diagnostics (all isDev-gated):
  • console.warn when recovery fires, naming the lost element and the recovery ancestor, with element refs attached and a pointer to printFocusHistory().
  • The Config.keyDebug "no event handler" log now prints the focus path leaf → root like a stack trace.
  • useFocusManager warns once if nothing is focused 1s after startup (missing autofocus).

Notes for reviewers

• When recovery fires, the destroyed element still receives its normal onBlur/onFocusChanged(false) callbacks via updateFocusPath. Blur handlers that touch the node should tolerate it being destroyed — same exposure as any late callback, but worth knowing.
• Deliberately out of scope: Row/Column selected reconciliation on child removal, focus traps, and a WebGL-drawn focusDebug overlay (the current overlay is DOM-renderer-only).

Testing

New tests/focusRecovery.test.tsx covers: recovery to parent (and that the dev warning fires), deep-subtree removal skipping detached ancestors, recovery re-running forwardFocus so a sibling gains focus, removal of a non-focused element not moving focus, and the focusLossRecovery = false opt-out. Full suite passes (139 tests), tsc clean, no new lint warnings.

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