feat(audience): Json.Serialize pretty-print overload + Samples.SampleApp IVT (SDK-231)

[ImmutableJeffrey]

Summary

• Json.Serialize(dict, indent) pretty-print overload — indent ≤ 0 routes to the compact Serialize, so every wire-payload caller is unaffected.
• InternalsVisibleTo("Immutable.Audience.Samples.SampleApp") so the in-repo sample reaches Json.Serialize / JsonReader.DeserializeObject without widening the public API.
• Audience.Tests.csproj excludes Editor/**/*.cs from the headless dotnet build — no-op today, prereq for the sample-app PR whose editor-only tests reference Unity types.

Closes SDK-231. Prereq for SDK-230 (Unity Audience SampleApp) landing next.

Test plan

• dotnet test src/Packages/Audience/Tests/Audience.Tests.csproj passes.
• Unity Test Runner (2022 LTS+): existing JsonTests fixture still passes; pretty-print round-trips through Deserialize identically.

🤖 Generated with Claude Code

---

Note

Medium Risk
Touches core JSON serialization used for event storage/network payloads; while the default compact path is intended to be unchanged, regressions could affect message formatting/round-tripping. Build changes also alter which tests compile in headless CI, potentially hiding Unity-only failures outside the Unity runner.

Overview
Adds a new Json.Serialize(dict, indent) overload to optionally pretty-print JSON with indentation, while keeping the existing Serialize(dict) as the compact default for wire/disk payloads.

Refactors the JSON writer to thread indent/depth through object/array/value writing and insert newlines/spaces only when pretty-printing. Also grants InternalsVisibleTo for Immutable.Audience.Samples.SampleApp and excludes Editor/**/*.cs from the headless Audience.Tests build to avoid Unity-only compile dependencies.

^{Reviewed by Cursor Bugbot for commit 73d3e46. Bugbot is set up for automated code reviews on this repo. Configure here.}