feat(tui): live Markdown rendering during text streaming (v0.9.0)

Replaces v0.8.2's drop-Markdown patch with proper in-place Markdown
rendering. The transcript becomes a VerticalScroll container; each
turn's response body lives as a single Static widget whose content
is updated as Text deltas arrive — Markdown is re-rendered in place
rather than re-printed on Done. Eliminates the v0.8.x double-print
without sacrificing rich formatting.

- transcript: RichLog → VerticalScroll (#transcript-scroll)
- Text deltas: mount Static(Markdown(buffer)) on first delta;
  Static.update(Markdown(buffer)) on subsequent deltas
- --raw mode: bypass Markdown, mount Static(plain_str) for the same
  in-place update semantics
- Terminal events (Done/Error/Cancelled) mount styled label Statics
- _cancel_via_sse: write → mount Static on the new container
- _write_turn_headers: transcript gets a styled RichText Static
  ("── turn N ──"); other panes still receive Rule renderables
- Test suite reshape: bulk rename `log` → `transcript` for the
  presenter contract, `_mounted_renderables` helper extracts
  Static.content for assertion, `_spy_writes` captures both
  RichLog.write and VerticalScroll.mount

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "ratatoskr"
-version = "0.8.2"
+version = "0.9.0"
 description = "Worldtree Conversation API debug TUI — multi-pane observability dashboard"
 readme = "README.md"
 requires-python = ">=3.12"