[TS Calls] Record unknown-call decisions

[CaelmBleidd]

Summary

• record a structured TsUnknownCallEvent for modeled, fresh-symbolic, and stopped unknown calls;
• attach the applied model ID or residual fallback policy and reason;
• preserve immutable path-local event history across ordinary forks and model-provider forks;
• expose a minimal observer whose failures are isolated from symbolic exploration;
• reject observer configuration that a custom dispatcher would silently bypass.

Scope boundary

This PR intentionally does not add stopped-path frontier snapshots, textual path constraints, frontier capture options, or frontier-specific callbacks. That research-only API will be designed with its concrete article consumer and is tracked separately in #373.

Builds on #372.

Testing

• ./gradlew :usvm-ts:test --rerun-tasks :usvm-core:detektMain :usvm-core:detektTest :usvm-ts:detektMain :usvm-ts:detektTest
• git diff --check origin/main..HEAD

Closes #364.
Part of #360.