docs(audience-sample): list tested operating systems (SDK-317)

- Adds a "Supported Platforms" section to the audience sample app README.
- Lists the operating systems the sample app has been tested on: Windows 10,
  macOS Sequoia 15.7.4, and Linux Ubuntu 22.04 LTS.
- No script porting required. The audience sample app ships no .ps1 or .sh
  files; the audience-sampleapp path named in the original ticket does not
  exist. The sample lives at examples/audience and uses Unity Editor C#
  builders.

Closes the remaining SDK-317 Definition of Done items.

Linear: SDK-317
https://linear.app/imtbl/issue/SDK-317/get-the-audience-sdk-to-build-and-run-on-linux

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

examples/audience/Assets/SampleApp/README.md

@@ -7,6 +7,12 @@ A demo of the `ImmutableAudience` SDK that exercises every public API.
 - Unity 2021.3 LTS or later.
 - An Immutable publishable key (`pk_imapik-test-…` for sandbox, `pk_imapik-…` for production).
 
+## Supported Platforms
+
+- Windows 10
+- macOS Sequoia 15.7.4
+- Linux Ubuntu 22.04 LTS
+
 ## How to use
 
 1. Open `Scenes/SampleApp.unity` and press Play.