docs: resolve open documentation issues

[7shep]

Summary

Resolves the six open issues labeled documentation with documentation grounded in the current contracts and repository scripts:

• adds a composition-library guide covering ownership, deterministic selection, compatibility, signatures, history, multi-page contracts, and validation
• adds a design-linter guide covering review evidence, result semantics, extension points, package boundaries, and current limitations
• documents the shared Button API, native prop forwarding, default type="button", accessibility expectations, and package ownership
• turns the Axis 65 README into a guided run, MCP planning, redesign, and implementation-review walkthrough
• adds a cross-linked glossary for Universal's design, provenance, and review terminology
• adds cross-platform setup guidance for version checks, Corepack/pnpm, PowerShell execution policy, filtered development, strict Vite ports, and validation

Closes #46
Closes #47
Closes #48
Closes #49
Closes #50
Closes #51

Why

The public APIs and workflow terminology were discoverable only by reading source and large technical documents. Setup and demo instructions also omitted common platform and port failures. These guides keep package-local behavior close to its implementation while linking back to the authoritative contracts.

Issue #49 referred to end-to-end generation as unimplemented. Current main includes the Phase 3 generation/runtime flow, so the updated demo guide accurately distinguishes editing the checked-in demo from generating a new immutable runtime-owned project instead of preserving that stale claim.

Validation

• pnpm format:check
• pnpm lint
• pnpm typecheck — 15/15 workspaces passed
• pnpm build — 15/15 workspaces passed
• pnpm test — 13/13 workspaces passed; one existing Windows junction test skipped by its platform guard
• git diff --check
• repository-wide local Markdown link check — 29 files checked, all targets exist

Setup verification environment

• Windows 11
• Windows PowerShell 5.1
• Node.js v22.20.0
• pnpm 11.7.0 via Corepack

The POSIX commands and scripts were checked against repository configuration but were not executed on macOS or Linux; the setup guide labels that limitation explicitly.