feat(audience-quickstart): add drop-in QuickStart sample (SDK-229)

[ImmutableJeffrey]

Summary

Adds a Samples/QuickStart folder containing AudienceDemo.cs — a drop-in MonoBehaviour that exercises every public ImmutableAudience API from an IMGUI overlay. Studios can set a publishable key, press Play, and verify their audience pipeline end-to-end without wiring a Canvas or prefab.

IMGUI overlay

• Boxed SDK-status panel reads the public diagnostic getters live (Initialized, Environment, Consent, Pub key, UserId, AnonymousId, SessionId, Queued). Full GUIDs render without truncation; a Copy button next to each ID writes the raw value to the system clipboard. The Pub key row highlights the REPLACE_ME placeholder in red so misconfiguration is obvious before the backend returns 401.
• Sections: SDK lifecycle (Init / Shutdown / Flush), Typed events, Custom event, Identity, Privacy consent, Advanced (GDPR erasure). Each section has a plain-English description so a first-time reader understands what the buttons do without opening source.
• Two-line buttons: action sentence on top ("Player started a level"), SDK API in parens below ("(Progression.Start)").
• Buttons gate on live SDK state. Track-family disables below Consent.None; Identify / Alias require Consent.Full; each Consent button disables at its own level; Init enables only when the SDK is not initialised; Shutdown / Flush / Delete enable only when it is.
• No auto-init. The user presses "Start the SDK" explicitly; every SDK-dependent button stays disabled until they do.
• Panel width caps at 800px and centres horizontally so the overlay reads as a focused card on wide screens and fills narrow Game views without an off-to-one-side sidebar.

Bundled scene

• AudienceDemo.unity is a minimal scene with a camera, a light, and one GameObject carrying AudienceDemo. The overlay renders entirely through IMGUI so the scene has no dependency on a specific uGUI or UI Toolkit version.

Sample payloads

• Familiar fantasy terminology (overworld, stone_age, gold, diamond_sword, dragon_defeated) — cold-readable without gaming background. Studios replace these strings with their own world names, currencies, item ids, and milestones during integration.

Transport fixes (bundled)

• HttpTransport reports backend response body on 4xx so the validation message reaches onError instead of being dropped.
• Request envelope corrected from `{"batch":[…]}` to `{"messages":[…]}` to match the backend MessagesRequest schema.

Linear: SDK-229

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Note

Medium Risk
Touches the event flush transport by changing the request envelope to {"messages":[...]} and surfacing 4xx response bodies, which could impact backend compatibility and error reporting behavior if assumptions differ. Sample additions are low risk but increase package surface area.

Overview
Adds a drop-in QuickStart Unity sample (Samples/QuickStart) including an AudienceDemo IMGUI overlay and a minimal .unity scene, letting studios initialize the SDK, send representative events, manage identity/consent, and trigger flush/GDPR deletion without building UI.

Updates HttpTransport to (1) wrap queued events under {"messages":[...]} instead of {"batch":[...]} and (2) include the backend’s 4xx response body (truncated) in ValidationRejected errors for more actionable diagnostics; associated unit tests and gzip test payloads are updated accordingly.

^{Reviewed by Cursor Bugbot for commit 99bf8c2. Bugbot is set up for automated code reviews on this repo. Configure here.}

[nattb8]

Is it supposed to be Samples~/