[codex] Preserve route topology declaration order

[ScriptedAlchemy]

Summary

• make route topology snapshots preserve route-manifest insertion order
• encode route ordinal positions so sibling reorder triggers topology changes
• update route-watch tests for React Router sibling declaration-order semantics

Context

This follows up on the Slack design note in /home/zack/Downloads/react_router_request.md. Most of that note describes a future in-place route topology design or items already covered by PR #46; this PR addresses the remaining small current-baseline correction that route order is semantic.

Verification

• pnpm test:core -- tests/route-watch.test.ts
• pnpm build
• pnpm format:check
• git diff --check