perf(editor): reduce default editor bundle size

[cdskill]

Summary

• move the table plugin to the @qalma/editor/table secondary entry point so prosemirror-tables stays out of the main entry
• annotate plugin factory exports and add a guard test so unused plugins remain tree-shakeable
• refresh bundle-size docs and benchmark results for the local 0.1.0-beta.2 candidate

Validation

• pnpm nx build editor
• pnpm nx test editor --runInBand
• pnpm exec tsc -p libs/editor/tsconfig.lib.json --noEmit
• pnpm exec tsc -p apps/docs/tsconfig.json --noEmit
• pnpm guard:protected-test-changes
• npm run bench from bench/bundle-size