[TS Calls] Record unknown-call decisions

[CaelmBleidd]

Summary

• extend the existing TsInterpreterObserver with onUnknownCall(event);
• report structured decisions for modeled, fresh-symbolic, and stopped unknown calls;
• attach the applied model ID or residual fallback policy and reason;
• isolate observer failures from symbolic exploration;
• cover all outcomes and model-created forks with focused tests.

Scope boundary

This PR is local to usvm-ts. It intentionally does not add a separate observer API, persistent history to TsState, generic StepScope helpers, stopped-path frontiers, textual path constraints, or frontier capture options. The research-only path-history/frontier contract will be designed with its concrete article consumer and is tracked in #373.

Builds on #372.

Testing

• ./gradlew :usvm-ts:test --tests org.usvm.machine.call.TsUnknownCallDispatcherTest
• ./gradlew :usvm-ts:test --rerun-tasks :usvm-ts:detektMain :usvm-ts:detektTest
• git diff --check origin/main..HEAD

Closes #364.
Part of #360.