chore: switch build backend to flit and bump version to 0.3.0

Move from uv_build to flit_core with dynamic version and description. flit
derives both from the package module, so add a module docstring (→ description)
and `__version__` (→ version) to src/ymprint/__init__.py, and declare
`dynamic = ["version", "description"]` in pyproject.

Bump to 0.3.0 for release: several backward-compatible features have merged
since the 0.2.0 release (multi-page templates, scoped text styles, explicit
list syntax, graceful CLI error handling).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: connor-sp

pyproject.toml

@@ -1,7 +1,6 @@
 [project]
 name = "ymprint"
-version = "0.2.0"
-description = "Desktop publishing in YAML with Python"
+dynamic = ["version", "description"]
 readme = "README.md"
 authors = [
     { name = "Connor Ferster", email = "connor@structuralpython.com" }
@@ -23,8 +22,11 @@ dependencies = [
 ym = "ymprint.cli.main:app"
 
 [build-system]
-requires = ["uv_build>=0.11.16,<0.12.0"]
-build-backend = "uv_build"
+requires = ["flit_core>=3.9,<4"]
+build-backend = "flit_core.buildapi"
+
+[tool.flit.module]
+name = "ymprint"
 
 [dependency-groups]
 dev = [

src/ymprint/__init__.py

@@ -1,2 +1,6 @@
+"""Desktop publishing in YAML with Python."""
+
+__version__ = "0.3.0"
+
 from ymprint import config
-from ymprint import blocks
+from ymprint import blocks