feat(core): v0.3.0 observability release

[seanbrar]

Summary

Add exception tracking in scopes, a LogReporter for structured logging output, and fix three SimpleReporter rendering bugs: parent scope clobbering in the timings tree, gauges incorrectly displayed as counters, and as_dict() returning tuples instead of dicts.

Related issue

None

Test plan

8 new tests in tests/test_core.py covering all changes (23 total, all passing):

• test_report_shows_nested_and_parent_scopes — verifies parent scopes with own data render as stats lines, not just tree headers (the clobbering bug)
• test_report_shows_structural_headers — verifies intermediate-only nodes (e.g., a: when only a.b.c recorded) get visual header lines
• test_as_dict_returns_dicts_with_named_keys — verifies entries use {"duration": ..., **meta} / {"value": ..., **meta} instead of tuples
• test_report_gauge_shows_last_value — verifies gauge metrics display Last: instead of Total:
• test_exception_metadata_recorded_on_scope_exit — verifies error, error_type, error_message are captured and exception propagates
• test_exception_metadata_absent_on_success — verifies zero cost on happy path (no error keys in metadata)
• test_log_reporter_emits_timing_record — verifies log level, message format, and extra dict structure

Benchmarks show no performance regression — the only hot-path change is a single if exc_type is not None: branch in _Scope.__exit__, not taken on the happy path.

uv run ruff check .           # All checks passed
uv run mypy src tests         # No issues found
uv run pytest -q              # 23 passed
uv run python benchmarks/bench_core.py  # No regression

Notes

• Flat sorted rendering replaces tree rendering for timings: _build_hierarchy + _format_tree replaced by a single _format_timings method. Sorted scope names naturally put parents before children; structural headers provide visual grouping without the clobbering bug.
• Scope tags/labels from the v0.3 roadmap was deferred — **metadata already covers the use case.
• LogReporter uses nullscope.telemetry logger (not nullscope) to avoid colliding with the library's internal logger. Default level is DEBUG since telemetry is high-volume.
• Exception tracking uses __qualname__ for error_type to correctly handle nested classes.

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• PR title follows the conventional commits style
• make check passes
• Tests cover the meaningful cases, not just the happy path
• Docs updated (if this changes public API or user-facing behavior)