Decouple structured events from stdlib logging via explicit EventSink

[marcleblanc2]

Implements Track B (lib side) of the file-handling + observability redesign
(spec: src-auth-perms-sync dev/PLAN.md).

What changed

• New events module: EventSink protocol with Null/InMemory/Callback/
  Composite/JSONL sinks and an EventRuntime ambient run scope (default:
  null sink — importing the library never writes anywhere).
• OTel Logs Data Model event schema: time_unix_nano, severity_text/
  severity_number, event_name, body, trace_id/span_id/
  parent_span_id, nested attributes, resource stamped once per run.
• Wide-event discipline: the span-end event is the canonical wide event;
  span-start events demoted to debug; error_type → error.type (semconv).
• Module-friendly logging: observability_context() owns one run without
  touching stdlib handlers. cli_logging_handlers() attaches terminal +
  EventBridgeHandler to named package loggers only — never the root
  logger, no handlers.clear() (fixes host applications having their
  logging config destroyed on import). cli_run_context() composes both
  with the JSONL sink for CLI entrypoints.
• Carried over: httpcore wire-debug mining, HTTP header secret
  redaction, exc_info tracebacks (now in the bridge), log-file retention
  pruning, resource sampler, run summaries.
• HTTP run-summary counters renamed to http.client.* aligned names;
  metric reset moved into observability_context (per-run correctness for
  long-running hosts).
• OTLP logs: logs provider configured when exporting; OtelLogsSink
  emits wide events through the OTel Logs API.
• Deleted: configure_logging root clearing, JSONLogFileHandler,
  _DropStructuredEvents, structured-record magic attrs.

Verification

• uv run python -m unittest discover -s tests — 104 tests OK
• uv run pyright — 0 errors (strict, src + tests)
• uv run ruff check / format --check — clean
• Consumer (src-auth-perms-sync) developed against this branch: full fast
  suite 102/102 and live e2e 119/120 against the sgdev test instance (the
  one failure is the wheel smoke needing this release on PyPI).