cua-driver installer: print the PATH export command when ~/.local/bin isn't on PATH

Summary

When the installer finishes and the install dir (~/.local/bin by default) is not on PATH, the post-install output never prints the copy-pasteable command to add it. The user is left with a working binary they can't invoke, and no instruction to recover.

I hit this on a fresh macOS install: cua-driver 0.19.2 was installed correctly, the symlink was in place, but which cua-driver returned nothing and nothing in the install output told me how to fix it.

Repro / evidence

$ which cua-driver
cua-driver not found

$ ls -la ~/.local/bin/
lrwxr-xr-x  1 administrator  staff  53  cua-driver -> /Applications/CuaDriver.app/Contents/MacOS/cua-driver

$ /Applications/CuaDriver.app/Contents/MacOS/cua-driver --version
cua-driver 0.19.2          # binary is fine

$ case ":$PATH:" in *":$HOME/.local/bin:"*) echo YES;; *) echo NO;; esac
NO

~/.zshrc existed but contained no PATH line, and ~/.zprofile / ~/.zshenv / ~/.profile didn't exist at all — so ~/.local/bin was never going to be picked up.

Where the gap is

libs/cua-driver/scripts/_install-rust.sh:1064-1080:

# Auto-extend PATH for users whose shell doesn't already include BIN_DIR.
if [[ "$NO_MODIFY_PATH" != "1" ]] && [[ ":$PATH:" != *":$BIN_DIR:"* ]]; then
    SHELL_RC=""
    case "${SHELL:-}" in
        */zsh)  SHELL_RC="$HOME/.zshrc"  ;;
        */bash) SHELL_RC="$HOME/.bashrc" ;;
    esac
    if [[ -n "$SHELL_RC" ]]; then
        ...
        log "appended PATH update to $SHELL_RC — open a new shell or run \`source $SHELL_RC\`"
    else
        log "WARNING: $BIN_DIR is not on PATH; add it manually."
    fi
fi

Three ways a user ends up with no actionable command:

1. --no-modify-path / CUA_DRIVER_NO_MODIFY_PATH=1 prints nothing at all. The whole block is gated on NO_MODIFY_PATH != 1. Opting out of the rc edit shouldn't also opt you out of being told what to do — that's exactly the user who most needs the line.
2. The else branch says "add it manually" without saying what to add. Any shell that isn't zsh/bash (fish, nushell, ksh) or an unset/odd $SHELL (piped installs, CI, sh -c, provisioning scripts) lands here.
3. The success path only names the rc file, never the line itself. If the append doesn't take effect — rc not sourced for login shells, a .zprofile-driven setup, dotfile manager later overwriting .zshrc — there's nothing to copy. This is likely what happened to me.

Requested change

Whenever BIN_DIR is not on PATH, always print the explicit command, independent of NO_MODIFY_PATH and independent of whether shell detection succeeded. Something like:

warning: /Users/you/.local/bin is not on your PATH.

  Add it with:
    echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.zshrc && source ~/.zshrc

  Or run the binary by full path:
    /Users/you/.local/bin/cua-driver --version

Suggested shape:

• Compute the hint once and print it in all branches — appended, not-appended, and opted-out. When the rc edit did happen, still show the line so the user can verify or re-apply it.
• Emit correct syntax per shell, falling back to POSIX when $SHELL is unknown:
  • zsh/bash → export PATH="$BIN_DIR:$PATH"
  • fish → fish_add_path $BIN_DIR
  • nushell → $env.PATH = ($env.PATH | prepend "…")
• Make it visually prominent — this is the difference between a working install and an apparently broken one, so it deserves more than a log line buried above the cua-driver X installed. banner.

Consistency across the other installers

The comment at _install-rust.sh:1096-1101 notes that post-install hints live in a shared post-install-hints.txt so the four Rust installers don't drift. The PATH hint is currently not part of that shared text, and the same gap exists in the siblings:

• libs/cua-driver/scripts/install.ps1 — -NoPathUpdate
• libs/cua-driver/scripts/install-local.ps1 — -NoPathUpdate ("skip the auto-append of the bin dir to the User PATH")
• libs/cua-driver/scripts/_install-local-rust.sh — has no BIN_DIR-not-on-PATH check at all

Worth fixing in one pass so all four behave the same. The Windows ones need the setx / $env:Path equivalent, and a note that an already-open terminal won't see a User PATH change until restarted.

Environment

• macOS (Darwin 25.6.0), Apple Silicon, zsh
• cua-driver 0.19.2
• Default BIN_DIR (~/.local/bin)