formatString interpolation

[23min]

Spec reference

• A2UI v0.9 Evolution Guide

Current behavior
Data binding supports literal values, path-based resolution, and combined literal + path. No support for formatString function.

Required behavior
v0.9 introduces a formatString function that supports ${expression} interpolation syntax in bound values. For example: "Hello, ${/user/name}!" should resolve the path /user/name from the data model and interpolate it into the string.

Affected files

• src/A2UI.Blazor/Services/SurfaceManager.cs — data binding resolution logic
• Possibly a new FormatStringResolver service

SPECIFICATION.md reference

• Data Binding: formatString interpolation row

Acceptance criteria

• Implementation matches spec
• Tests added (red-green-refactor)
• SPECIFICATION.md updated (gap → compliant)
• Sample servers updated if affected (parity)
• Sample apps updated if affected (parity)

[23min]

Completed in PR #17