feat: keyframe storytelling and verified WebR compatibility

[mortonanalytics]

Closes #89
Closes #90

Summary

• add additive R and Shiny keyframe APIs with complete transformed snapshots
• add accessible one-pass keyframe controls using the existing in-place update path
• add a pinned WebR 0.6.0 Wasm build, payload-transfer, and Chromium-render release gate
• update schemas, release tooling, package exclusions, README, NEWS, and sequential-storytelling documentation

Engine contract

This is an additive canonical-engine change visible to Python consumers. No existing config defaults, event names, aliases, or specVersion = 2 behavior are removed. The pymyIO submodule should pick up the merged engine through the automatic tracking issue.

Verification

• devtools::test(): 1,249 pass, 3 fixture-dependent skips
• Vitest: 419 pass
• Playwright: 26 pass
• pkgdown site build: pass
• WebR 0.6.0 local official-action build + library(myIO) + payload transfer + production bundle Chromium render: pass
• MCP dependency transition was reviewed separately in build(deps): bump @hono/node-server and @modelcontextprotocol/sdk in /mcp #99

Compatibility

• additive engine surface
• no pymyIO code changes in this PR
• WebR claim is bounded to the pinned precompiled package path; DuckDB-WASM and every browser host are not claimed