fix: keep cursor at vertical motion boundaries

[s3onghyun]

Normal-mode k on the first line and j on the last line should fail without moving the cursor, matching Vim/Neovim. Today the cursor jumps to the start/end of the line instead.

Keep the existing linewise operator behavior so dj/dk at a document boundary still select the expected lines.

Adds a regression test for both normal-mode boundaries (top k and bottom j preserve a nonzero column). types.ts now declares moveByLines as returning Pos | null to match.

Verified locally: pnpm run test (712 passing) and pnpm run testAll (CM6 712, CM5 708, package and TypeScript consumer builds).

Context: I maintain a small Vim/Neovim trainer that checks this emulator against headless Neovim 0.11 and currently carries this as a local patch; happy to adjust to your preferred approach.