chore: bump version to 0.2.4

[thesmallstar]

Summary

• Bumps version in pyproject.toml from 0.2.3 to 0.2.4.
• Refreshes uv.lock to track the new version.

What ships in 0.2.4

This release includes the nested-list spacing/indent overhaul merged in #20:

• Adds proper 4-space indent per nest level for bullet and ordered lists.
• Adds bullet hierarchy: • / ◦ / ▪ for depths 1 / 2 / 3+ (matches Slack's native rendering).
• Fixes multi-blank-line cascades from deeply nested list closes and blockquotes (was producing 2+ blank lines between blocks; now exactly one).
• Preserves multi-blank-line content inside fenced code blocks verbatim.
• Adds 10 new edge-case tests + 1 full-document integration test (60 total).
• Adds docs/architecture.md documenting the renderer pipeline and the STX-sentinel cap mechanism.

Test plan

• pytest tests/ — 60/60 pass locally