fix(ui): follow toolkit's MVVM-only widgets, unblocking the upgrade to Android

[tannevaled]

go-widgets/toolkit converted 20+ widgets in one sweep (v0.189 → v0.204) from "settable field plus an OnChange slot" to "state is an mvvm.Observable, reached through an accessor". reader pinned toolkit v0.188.0, so nothing was broken for its users — but it could not move, and 34 compile errors stood between it and any newer toolkit.

Why that blockage now costs something

go-widgets/window v0.44.0 wires in the Android back-end: Open dials the go-widgets/android host inside an APK, and falls through to X11/Wayland under Termux. reader gets Android by upgrading — once it can upgrade.

It builds for android/arm64 as of this commit, and the cross-build matrix gains that target so the capability is CI-guarded rather than proven once by hand.

The port

Four APIs moved; each call site follows to its accessor.

API	was	now
SearchEntry.Text	field	Text() *Observable[string]
TreeView.ScrollRow	field	ScrollRow() *Observable[int]
TreeView.Selected	field	Selected() *Observable[*TreeNode]
Spinner.Active	field	Active() *Observable[bool]

.Text is not a safe blanket rename — non-converted widgets still have a Text field — so the sweep is by identifier, not by symbol.

app/bind.go is the one non-mechanical change

An MVVM-only widget has no field to seed and no callback to compose, so BindField cannot express it. What it has is a property that must agree with the ViewModel's — which is mvvm.BindTwoWay, added in mvvm v0.8.0 for exactly this shape:

mvvm.BindTwoWay(vm.Search, s.SearchEntry().Text(), s.InvalidateSearch)

The reasoning about which thread may Set these is unchanged and still documented at the call site.

TestSearchEntryWidget stood in for the binder by filling the OnChange slot; it now subscribes to the widget's own Observable, which is what the binder actually does.

Gate

gofmt clean · go vet clean · 100.0% coverage · -race clean · window boundary green · every cross-build target including android/arm64.

js/wasm still fails on modernc.org/sqlite exactly as it does on main (verified against origin/main as a control) — pre-existing, and not in the matrix.

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