Consume a ThemeContribution's Styles as a library selector-style leg

[mstrobel]

Problem

A control library could contribute Type-keyed control themes + resources via ThemeContributions, but its ResourceDictionary.Styles selector channel was explicitly not consumed (§11.8). So a library had no way to ship app-level selector styles — only implicit, ControlThemeKey-resolved themes. Adding arbitrary styles to a contribution silently did nothing.

Fix

The StyleEngine now gathers each registered contribution's top-level Styles as a third leg of the Theme layer, ordered:

• above the BuiltIn framework leg (a library refines the framework default), and
• below the app.Theme leg and UIApplication.Styles (the app always wins).

Each contribution gets a distinct 4096-wide order slot in registration order, so a later-registered library wins a same-target tie (the resource-tier "last wins", applied to styles); the slot is clamped one stride below AppThemeOrderBase so a contribution can never reach the app-theme leg no matter how many register.

• A contribution shipping only Type-keyed themes (no top-level Styles) is skipped; the whole leg short-circuits when nothing is contributed — zero cost in the common case.
• Late registration (rare — contributions normally register at module init, before any app) re-arms the engine app-wide via OnThemeStylesInvalidated, alongside the existing resource catch-all.
• Only the dictionary's own top-level Styles are consumed; Styles nested in a contribution's MergedDictionaries are not flattened (v1) — same contract as the app.Theme leg. The ancestor-interesting fast path leaves the whole Theme layer out (BuiltIn + app.Theme + contributions alike), unchanged.

On the type-resolver question

This path needs no ISelectorTypeResolver: a library contributing styles in code builds typed selectors — Selectors.OfType(typeof(BarButton)) / Selectors.Is<Ribbon>() — no string parsing. The resolver plumbing (for string/XAML-authored selectors) stays a separate, deferred concern.

Docs / tests

Reversed the §11.8 note on ThemeContributions and updated ResourceDictionary.Styles. Section15_ContributedStyles covers match+apply, App overrides, app.Theme overrides, later-contribution-wins, no-Styles-skipped, and late-registration re-arm — with a global-registry save/restore hook for isolation. Mutation-verified (removing the leg fails the match test). UI 2957 / Bars 296 green.

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