input: preserve CRLF cursor boundaries and document shortcuts

[kossoy]

End can place a text-input cursor between CR and LF; subsequent typing splits the newline. Ctrl+Home/End also lacked non-macOS document bindings.

Clip cursor and selection movement outside CRLF pairs, including the new multi-cursor movement/selection paths. Left/Right cross CRLF as one newline; a standalone CR remains an ordinary character. The rope and byte/UTF-16 APIs are unchanged. Add standard document navigation/selection bindings and a focused regression.

Validation on this branch (43c733d), adapted to the multi-cursor implementation in cbdf5ba:

• Linux and Windows full workspace test jobs pass; the Linux log confirms textarea_cursor_treats_crlf_as_one_newline passes among 838 base tests.
• macOS lint, workspace tests and component doc-test step pass.
• GPUI Shell Core, Component Shell, all three Standard Runtime jobs and documentation build pass.
• Changed Rust files pass rustfmt checking locally.

The pinned-version counterpart additionally passed native GPUI key-dispatch tests for LF/BOM/CRLF/mixed input, exact source-write payloads and actual muninn Linux Ctrl+End typing/save plus Ctrl+Home/End/Right/Left save checks in Tessera. Canonical BOM and all eight original CRLF pairs were retained.

CI run

[huacnlee]

Thanks