feat: add requireChildren option for route matching

[uhyo]

Summary

• Add a new requireChildren option to route definitions that controls whether a parent route can match when no children match
• Default behavior is true (breaking change): parent routes now require at least one child to match
• Setting requireChildren: false allows parent routes to match alone with outlet as null
• This enables catch-all NotFound routes to work intuitively for unmatched nested URLs

Behavior

requireChildren	No child matches	Result
true (default)	—	No match (falls through to next route)
false	Parent has component	Match parent, outlet=null
false	Parent has no component	No match

Test plan

• Unit tests added for requireChildren option in matchRoutes.test.ts
• Updated existing tests that relied on old behavior to use requireChildren: false
• All tests pass (pnpm test:run)
• Type checking passes (pnpm typecheck)

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