build: split graph-compose-render-pptx out of render-docx

[DemchaAV]

Why

graph-compose-render-docx carried both the DOCX backend (Apache POI) and the
semantic PPTX backend. Splitting PPTX into its own module lets a consumer who wants
only slide output avoid pulling POI, and keeps each backend's dependency surface
minimal. The PPTX backend is a POI-free skeleton today (it validates slide-safe
nodes and returns an export manifest), so the new module depends only on
graph-compose-core.

What

• New graph-compose-render-pptx module — graph-compose-core only, no POI.
  PptxSemanticBackend moves here from render-docx with its package/FQN unchanged.
• Split the shared SessionSemanticExportTest: the docx half stays in render-docx,
  the pptx half becomes PptxSemanticExportTest in render-pptx (both assertions
  preserved).
• render-docx keeps the DOCX backend only; its description/comment updated.
• Wired into the aggregator, publish (deploy after render-docx),
  cut-release.ps1 (version bump + commit list), VersionConsistencyGuardTest
  (version lockstep), and the CHANGELOG.
• The CI build gate now also builds and tests render-docx and render-pptx — the
  render-docx tests weren't running in CI before (it's a reactor leaf, not an
  also-make upstream of the engine/testing/qa slice).

Tests

• Full reactor ./mvnw -f aggregator/pom.xml clean verify → BUILD SUCCESS, all
  modules; the new PptxSemanticExportTest passes and render-docx stays green after
  the trim.