ex: add conformance scaffolding and telemetry instrumentation

[deepfates]

Motivation

• Provide a tests-first foundation to verify Elixir runtime conformance against the language-agnostic tests.yaml spec.
• Enable deterministic, scripted LLM behavior for reproducible conformance runs via a FakeLLM adapter.
• Add runtime telemetry so internal lifecycle and gate/code execution events are observable without changing runtime semantics.

Description

• Add a small conformance scaffold: Cantrip.Conformance.TestCase, Cantrip.Conformance.Loader, Cantrip.Conformance.FakeLLM, Cantrip.Conformance.Expect, and Cantrip.Conformance.Runner under ex/lib/cantrip/conformance/ to load/validate tests.yaml, produce typed test cases, and execute simple cast-style actions.
• Implement a YAML loader (Cantrip.Conformance.Loader) that converts tests.yaml into Elixir data (uses a small Ruby-based YAML->JSON shim via System.cmd/2 to avoid adding Hex deps in this environment).
• Instrument runtime telemetry: emit [:cantrip, :entity, :start] / [:cantrip, :entity, :stop] in EntityServer, emit [:cantrip, :turn, :start] / [:cantrip, :turn, :stop] per turn with entity_id and turn_number, emit [:cantrip, :gate, :call] and [:cantrip, :gate, :result] with duration in Circle.execute_gate/3, and emit [:cantrip, :code, :eval, :start|:stop] around sandboxed code evaluation.
• Add ExUnit test files under ex/test/ to exercise the loader, FakeLLM, a small runner smoke test, and a telemetry assertion (M25TelemetryTest) that attaches a handler and asserts events/metadata were emitted.

Testing

• New ExUnit tests added: ex/test/conformance/loader_test.exs, ex/test/conformance/fake_llm_test.exs, ex/test/conformance_test.exs, and ex/test/m25_telemetry_test.exs (all tagged to exercise the new conformance scaffolding and telemetry).
• Confirmed tests.yaml contains 71 cases via a quick YAML count (ruby -ryaml -e ...) to match the loader expectation.
• Attempted cd ex && mix deps.get but dependency fetch was blocked in this environment due to network/SSL restrictions to Hex, so full dependency installation could not be completed.
• Attempted targeted mix test for the new tests but execution was blocked by a local Elixir version mismatch (project requires ~> 1.19 while the environment provides 1.18.3), so automated test runs could not complete here.

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Codex Task