Add runtime intelligence and dynamic level control

[nikhilvdev]

Introduces bounded FlightRecorderPlugin, AdaptiveSamplingPlugin, and RunSummaryPlugin for per-run debug capture, adaptive sampling, and run-level summaries. Adds runtime level reloading via signal/file/env watchers and documents the new behavior in the README and changelog. Includes focused tests and a memory benchmark for the flight recorder.

Summary

Checklist

• ruff check . passes
• mypy logquill passes (--strict on the public API)
• pytest passes, and new surface area has test coverage
• Public API additions are documented in README.md with a runnable example
• CHANGELOG.md has an entry under Unreleased
• If this touches the cross-language contract (levels, logger methods,
  record shape, transport/plugin contracts, config shape) shared with
  logquill-js, a matching tracking issue/PR is opened there

Scope