Add WebMCP authoring lifecycle and exact slide upserts

[ErickWendel]

Summary

• replace the legacy five-tool WebMCP surface with the refined 15-tool authoring catalog and explicit pending-capability responses for later tickets
• add create/state/operation lifecycle primitives with bounded state pagination and semantic-description freshness metadata
• add atomic, idempotent, multi-batch slide upserts with stable IDs, exact geometry, z-order, styling, media references, font resolution, explicit deletion, and ordered animations
• update the WebMCP showcase plus unit and browser contracts

Verification

• npm run lint (passes with one pre-existing unused eslint-disable warning)
• npm run typecheck
• npm test (799 editor tests passed, 2 skipped; all workspace suites passed)
• npm run build
• focused WebMCP Playwright contracts and showcase: 4 passed
• npm run test:e2e:coverage:webmcp: 1 passed with coverage report
• editor-contract coverage shard: 72 passed; unrelated PPTX mapper contract remains red because it expects #FFFFFF while runtime returns #111111, reproduced independently

Closes #171
Closes #172