Add client characterization suite and Reactive documentation

[s-cork]

Summary

• add a runtime-v3 consumer app and App Server/Playwright characterization harness with isolated cases and failure diagnostics
• characterize async effects, component lifecycle, bindings, writeback, Data Table Models, and server-driven updates
• add user documentation, concise LLM guidance, CI coverage, and a GitHub Pages deployment workflow
• record deferred Solid Signals 2.0 and major-version design work separately from public documentation

Known gaps

The suite records six expected failures: four overlapping async/pending behaviours, class-wide reactive_instance behaviour, and writeback after component removal. Unexpected passes fail CI so each improvement must be reclassified deliberately.

Validation

• CPython: 7 passed
• App Server/Playwright: passed with the six expected failures
• mkdocs build --strict
• git diff --check

Follow-up

The exhaustive low-level signal and collection matrix can be added separately. The high-risk Anvil-specific integration behaviour is covered here.

[s-cork]

This change is part of the following stack:

• Add client characterization suite and Reactive documentation #40 ◀

_{Change managed by git-spice.}