fix(installer): update watcher and remove archive hashing

[esmuellert]

Summary

• Remove the installer-side SHA-256 calculation for downloaded watcher archives.
• Update the bundled watcher release from 0.19.0 to 0.23.2.
• Update watcher installation and protocol fixtures.

Why

Neovim versions before 0.12 convert binary Lua strings containing NUL bytes to Blobs, while vim.fn.sha256() in those versions only accepts Strings. This caused automatic watcher installation to fail with E976 on supported Neovim versions such as 0.11.7.

The installer continues to use the fixed GitHub Release URL and retains its existing download, extraction, cleanup, and polling fallback behavior. Release-time checksum generation remains unchanged.

Closes #553

Testing

• tests/core/watcher_install_spec.lua: 2/2 on Neovim 0.13-dev.
• tests/core/watcher_install_spec.lua: 2/2 on Neovim 0.11.7.
• tests/core/watcher_protocol_spec.lua: 6/6.
• Real Neovim 0.11.7 smoke test downloaded and installed codediff-watcher v0.23.2 successfully.
• stylua --check and git diff --check passed.
• The full Lua suite had one unrelated local failure: ffi_integration_spec.lua found a locally built native library reporting version 3.1.1 while VERSION is 4.0.1.