Add Flask adapter

[fsecada01]

Overview

Add a Flask adapter so users can install component-framework[flask] and use server-side components in Flask applications.

Required components

Per the Adapter Contract:

1. Renderer subclass — adapters/flask.py implementing the Renderer interface from core/renderer.py, using Jinja2 (Flask's built-in template engine)
2. HTTP endpoint handler — a Flask view function (or Blueprint) that dispatches POST requests to the component registry
3. Optional WebSocket handler — using flask-sock or similar, following the same event protocol as the HTTP handler
4. Optional extras group in pyproject.toml:

   [project.optional-dependencies]
   flask = [
       "flask>=3.0",
   ]

5. Example — at least one working example in examples/flask_example.py
6. Docs — pdoc-compatible docstrings on all public symbols

Constitution gates

• Adapter code MUST NOT import from core/ using framework-specific types
• The [flask] extra MUST NOT pull in FastAPI, Django, or JinjaX
• All adapter tests MUST use pytest.importorskip("flask")

Context

Part of the optional-dependencies refactor (Issue #4, PR implementing branch 001-optional-deps).
Flask and Litestar adapters were deferred to post-0.3.0.

Closes #4 (partially — Litestar tracked separately in #6)