Add local-first trace viewer & CLI commands

[nikhilvdev]

Introduce a local-first trace viewer and related CLI features:

• Add TraceBuilder/TraceNode engine (logquill/trace_tree.py) to stream-reconstruct a run's span tree and produce annotated rollups (tokens/cost), keeping memory proportional to the run.
• Add web UI (logquill/webui.py) and static page (logquill/static/trace_viewer.html) with JSON APIs; support JSONL and SQLite sources.
• Extend CLI (logquill/cli.py) with trace/serve/dev subcommands, colorization and TTY-aware rendering.
• Add comprehensive tests and a gigabyte-scale memory benchmark (benchmarks/test_trace_memory.py).
• Update README and CHANGELOG with usage and design notes.

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