[Feature]: Complete x:Bind assignment and accessibility diagnostics

Is your feature request related to a problem? Please describe.

PR #50 Round 5 binding validation passes 37 cases with zero false positives, but three negative cases remain silent: assigning int to string (XB08), assigning bool to Visibility (XB09), and binding to an inaccessible private member (XB21). The current implementation intentionally permits broad built-in XAML conversions and separately models member accessibility, so these need precise treatment rather than stricter blanket assignability.

Describe the solution you'd like

Define and test the intended compiled x:Bind conversion rules for primitive-to-string and bool-to-Visibility assignments, then diagnose only combinations that the WinUI XAML compiler rejects. Ensure private/inaccessible members produce the existing unknown/inaccessible-member diagnostic while valid page-private event handlers and supported binding members remain available.

Add focused language-server tests and live integration probes for all three cases, including nullable and converter-backed controls to prevent regressions.

Additional context

Follow-up from PR #50 Round 5 battle testing. These were unchanged pre-existing coverage gaps and were not regressions in prerelease.290.

Start by reading PR #50 Round 5 and the existing language-server tests for compiled x:Bind validation. Run or add the live integration probes for XB08, XB09, and XB21, including nullable and converter-backed controls. Done means rejected assignments produce the intended diagnostics while valid private handlers and supported binding members remain available.