docs: update binary format and runtime documentation

[zharinov]

Summary

• Update binary format docs for header flags and linking
• Add WIP documentation for future features:
  • anchor-restrictions.md - Anchor placement validation design
  • bytecode-linking.md - Linking design for grammar verification
  • bytecode.md - Bytecode dump format specification
  • grammar-verify.md - Grammar verification design
• Update lang-reference.md with anchor placement rules
• Simplify runtime-engine.md (remove outdated sections)

Depends on: #194

Test plan

• Documentation review