feat: v0.4.0 — data features

[23min]

Summary

• FunctionCall — computed dynamic values (formatString, openUrl, required, etc.) usable in any property position
• TemplateChildList — data-driven children from data model arrays for container components
• CheckRule — input validation rules with condition (DynamicBoolean) and message for text_field, checkbox, slider
• Theme — surface theming via createSurface (primaryColor, iconUrl, agentDisplayName)
• sendDataModel — flag on surface to request client send full data model with every action
• Path-level updateDataModel — upsert/delete at individual JSON Pointer paths (encoder, server broadcast, socket returns)
• Debug renderer — updated for FunctionCall display, path ops, TemplateChildList, and theme application

162 tests, 0 failures. 4 new modules, 3 new test files, +1,255 lines.

Test plan

• mix ci passes (format + compile warnings-as-errors + 162 tests)
• mix run demo_server.exs — verify new features render in debug view at http://localhost:4000
• Inspect JSON output manually for wire format correctness

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