refactor(core): simplify telemetry setup and harden runtime log control

[mattwend]

Summary

• Simplify subscriber construction, decouple Tokio metrics from OTLP wiring, and tighten OTLP
  configuration validation.
• Make TelemetryGuard shutdown idempotent with best-effort drop teardown.
• Wrap installed OTLP filters in live reload layers so runtime filter updates take effect
  immediately.
• Serialize mutable filter state behind a single mutex to eliminate an inverted lock-ordering
  deadlock between stdout and OTLP updates.
• Update README and crate-level docs to reflect the new shutdown semantics, configurable
  Tokio metrics, and rt-multi-thread requirement.

Why

• Previous control-plane updates could target orphaned filter state and the router had a
  latent deadlock path; both are now covered by regression tests.
• Production setup is leaner, shutdown is more reliable on both current-thread and
  multi-thread runtimes, and users have clear guidance on supported configuration paths.

Tests

• New integration coverage: init_otlp.rs, init_otlp_idempotent.rs, init_otlp_log_control.rs,
  plus expanded init_log_control.rs and readme_common_config.rs for lifecycle and README-style
  configs.