[API] Query String Support

[ArielG-NV]

Resolves: #628
Partially addresses: #597

Changes:

• Add support for a QueryString to flashdreams via QueryStringUserInputEvent. Event received by first-step of a UILoop/ModelLoop.
• Added imgui-ui-query-string demo to test functionality.

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

Summary

WebRTC connections now pass the browser URL query string into the first loop input event. The PR also adds an ImGui demo that reads an RGB color from that query and uses it as the background.

• Adds query-string support to WebRTC input handling.
• Adds an RGB background demo driven by the browser URL.
• Documents and tests the new connection input.

Diagram

sequenceDiagram
    participant Runner as Session runner
    participant Window as WebRTC window
    participant Browser
    participant Server as WebRTC server
    participant Buffer as Event buffer
    participant Loops as UI and model loops
    Runner->>Window: Open session
    Browser->>Server: GET /healthz
    Server-->>Browser: Window is open
    Browser->>Browser: Create offer and gather ICE
    Browser->>Server: POST /api/webrtc/offer + query
    Server->>Server: Read raw_query_string
    Server->>Buffer: Add QueryStringUserInputEvent
    Server-->>Browser: Return SDP answer
    Runner->>Buffer: Read queued input on a loop tick
    Buffer-->>Loops: Give event batch to both loops

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_{Reviews (3) · Last reviewed commit: "Update flashdreams/flashdreams/runtime_v..."}

[ArielG-NV]

/ok to test 4f2676a