Monorepo split: separate pip distributions

[masenf]

Split the reflex repository into a monorepo producing separate pip distributions, so users only install what they need.

Proposed Packages

Package	Contents
reflex-core	Runtime, event system, compiler, rx.Field, Var system
reflex-html	Base HTML elements with Tailwind support
reflex-radix	Radix Themes component library
reflex-lucide	Lucide icon set
reflex-orm	Database/ORM functionality (see ENG-9181)

The top-level reflex package becomes a meta-package that depends on all of the above for backward compatibility.

Motivation

• Users who don't need a database shouldn't install SQLAlchemy/Alembic
• Users who want a different component library shouldn't be forced into Radix
• Smaller install footprint for production deployments and containerized apps
• Clearer separation of concerns for contributors

Tracking

Parent: ENG-9180 (Dependency Reduction & Monorepo)
Roadmap: #2727 — Section 4

[linear]

ENG-9185