ttl: emit runtime task lifecycle log events

[kdy1]

Summary

Logging currently emphasizes compile/cache stage events but does not emit the runtime task lifecycle event names required by the contract.

Evidence

Contract Evidence:

• docs/project-ttl-language.md:206
• cmds/ttlc/internal/compiler/service.go:496

Current Gap

Logging currently emphasizes compile/cache stage events but does not emit the runtime task lifecycle event names required by the contract.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Emit lifecycle events: task_scheduled, task_started, task_cache_hit, task_cache_miss, task_completed, task_failed.
• Include required structured fields (compile_stage, task_id, cache_key, cache_hit, invalidation_reason, worker_id, duration_ms, error_kind).
• Keep ANSI color/no-color behavior intact for operator-facing output.
• Provide deterministic event ordering guarantees where applicable.

Test Scenarios

• Successful execution path emits scheduled/start/completed events.
• Cache-hit path emits task_cache_hit.
• Failure path emits task_failed with error_kind.

Out of Scope

• External log shipping integrations.
• Metrics backend instrumentation.