feat: add defineRoute and defineRoutes declarative helpers

[olliethedev]

Summary

• Adds defineRoute(path, def, options?, routeMeta?): a declarative variant of createRoute that removes the per-route handler-closure boilerplate. page/loading/error components receive the route context (params, query) as props; loader/meta/extra receive it as their first argument, with extra caller args (e.g. AbortSignal) forwarded after it.
• Adds defineRoutes(routes, { pages? }): groups routes for createRouter with optional per-key page component overrides. Overrides on defineRoute routes are rebuilt so the replacement component still receives the route context; plain createRoute routes get their PageComponent swapped as-is.
• Adds react as a peer dependency (the helpers use createElement to bind context into components); bumps version to 1.3.0.
• README section documenting the new helpers.

Motivation: consumers like @btst/stack currently repeat a handler closure per route wiring PageComponent override lookup, loader, and meta (~6x per plugin). These helpers collapse that to one declaration per route while staying fully compatible with createRoute/createRouter.

Test plan

• 19 new unit tests in src/__tests__/define.test.ts: context injection into page/loading/error props, render-time prop merging, loader/meta/extra binding (sync + async + extra args), query validation passthrough (pass and fail), route-level metadata, createRouter interop (routeKey, params), page overrides for both defineRoute and plain createRoute routes
• pnpm build, pnpm lint, pnpm typecheck, pnpm test -- --run all pass locally (79/79 tests)

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