Add a test-only R8 shrink-survival module with consumer keep-rules

[OmarAlJarrah]

Problem

Nothing verifies the toolkit survives consumer-side shrinking (R8/ProGuard) — more important for a library than for an app SDK.

Proposed change

Add a test-only sdk-shrink-test (Java 8, unpublished — shadow + R8 stay out of every published module so zero-dep core is preserved): shadow-JAR core + okio + okhttp transport + jackson serde, run R8, and ship the consumer keep-rules needed. Wire into CI.

Acceptance

• Shrink test module runs R8
• Keep-rules documented/shipped
• CI job

Dependencies

• Blocked by Add CI workflows to enforce the build gates on every PR #70 — CI must exist to wire the shrink job
• Related Extract a build-logic convention plugin to remove duplicated publishing config #71 — build-logic convention plugin (new module config)

---

Priority: medium · Effort: large