fix: skip pathless routes with loaders during SSR

[uhyo]

Summary

• Pathless routes with loaders were rendering during SSR with data: undefined, violating the component's type contract (data: TData). Now matchRoutes treats them as non-matching when pathname is null (SSR), so they only render after hydration when loaders can actually execute.
• Sibling pathless routes without loaders still match normally, enabling fallback SSR shells.
• Updated SSR documentation to reflect the new behavior.

Test plan

• Added 4 new unit tests in matchRoutes.test.ts covering:
  • Pathless route with loader does NOT match when pathname is null
  • Sibling without loader matches when pathless route with loader is skipped
  • Nested pathless route with loader stops SSR matching at that level
  • Pathless route with loader still matches on the client (non-null pathname)
• All 174 existing + new tests pass
• Full typecheck passes across all packages

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