Create PLAN.md, CLAUDE.md, README.md #7

Write the initial project documentation.

PLAN.md:

• 15-phase development roadmap
• Test suite milestones (html5lib, Acid1, Acid2, Test262, WPT)

CLAUDE.md:

• Architecture overview and crate dependency map
• Build/test/lint/format commands
• Conventions: zero external deps, unsafe policy, pure Rust mandate
• JS engine architecture notes
• Workflow guide and tangled CLI cheat sheet
• Test harness locations

README.md:

• Minimal: what it is, how to build, how to run

This is done — landed in the initial commit.