feat: dogfood React Chat Messages in the native example - #1250

Adds an experimental native React frontend for Chat Messages on the canonical Message model page. It composes MessageList, Markdown, ChatInput, Reasoning and Citations using the private source-built candidates, while preserving the default Angular example.

The conversation remains busy until one saved-history read confirms the exact completed turn and prior transcript. Stop, uncertain creation, conflicting history, tools, interrupts or child state block continuation until an explicit new conversation. Thread-title generation updates metadata without leaking its tokens into the answer stream.

Validation uses the actual compiled Python graph with locked deterministic providers and the installed SDK over HTTP/SSE. It covers incremental code fences, two-turn continuation, cancellation, teardown, developer targets, and independent website navigation/reload behavior without model requests. Fresh source suites, production browser checks, all installed native suites, assembled routes, strict types and visual checks are recorded before publication.

Fresh verification:

• 1,065 source tests across all 21 native examples; 308 installed browser checks across the same fleet.
• Full production website: 281 passed, four expected skips, no failures or flakes.
• All 62 frontends assembled; 21 native HTTP/CSP/SPA routes and private fixture/control/sourcemap exclusions checked.
• Three packed candidates, 13 strict ESM/type exports, three isolated core exports, and 27 installed React consumer scenarios.
• Scoped lint and strict source/unit/browser types; registry, workspace and parity checks; desktop/mobile visual inspection.