refactor(router): decompose monolithic router into single-responsibility modules

[mohamedmansour]

What changed

Broke the ~1,800-line monolith router.ts into 8 focused modules and optimized the Rust-side route handler for faster SSR rendering.

Client — webui-router

New module	Responsibility
cache.ts	NavigationCache — LRU eviction + tag-based invalidation

router.ts is now a thin orchestrator (~80 lines of imports + wiring) that delegates all heavy lifting to these extracted modules.

Server — webui-handler (Rust)

• ProtocolIndex — pre-computes component bit-index, compiled route cache (CompiledRouteCache), and per-fragment component closures once per protocol load instead of per-request.
• CompiledRouteCache — caches parsed route template patterns (Vec<SegmentPattern>) via match_route_cached(), eliminating per-request parsing allocations in the route-matching hot path.
• data-ri attribute — matched SSR routes now emit a chain index (data-ri="N") so the client can bind routes in O(1) instead of DOM-walking.
• window.__webui bootstrap object — replaces scattered <meta name="webui-inventory"> and <script id="webui-chain"> tags with a single consolidated inline script containing chain, inventory, nonce, css, and styles.
• write_usize() — writes numeric data-ri values directly to the response writer, avoiding format! heap allocation.

Other updates

• DESIGN.md — updated to reflect the new SSR bootstrap contract, ProtocolIndex, partial response fields (allowedQuery, keepAlive, cacheControl), and hydration flow.
• docs/guide/concepts/routing.md — updated user-facing routing docs.
• webui-framework — adjusted template/element APIs for the new registration flow.
• Tests — updated unit and E2E tests across router, handler, commerce example, and routes/todo examples.

[mohamedmansour]

@copilot resolve the conflicts, fetch origin main, and merge it, then resolve conflicts.

[Copilot]

@copilot resolve the conflicts, fetch origin main, and merge it, then resolve conflicts.

Done. Fetched origin/main and merged it into chore/refactor-router (commit 6539aad).

The only manual conflict was in packages/webui-router/src/router.test.ts — the "fix flaky preload cache test" from main used the old storeCacheEntry/priv.cache API, while our refactored branch exposes NavigationCache.store(). Kept the HEAD version since our NavigationCache fully replaces the old API. All 261 Rust handler tests and 54 router unit tests pass after the merge.