Preserve existing ln destinations unless force is requested - #1288

ln(source, existingDestination) reports Destination file exists but then continues to unlinkSync() and replaces the destination anyway. Without -f, this can overwrite a regular file, replace an existing symlink, or delete the source itself when source and destination are the same path.

Stop the command at the existing-destination error. The returned failure status/message and explicit -f/-sf behavior are preserved.

Tests strengthen the existing destination check and add nine cases covering regular files, symlink destinations, missing sources, self-links, fatal mode and a dangling-link control.

Validation on Linux / Node.js 24.19.0:

• Fresh baseline: 640 passed / 6 existing skips, including post-test lint.
• Final tests against unchanged production code: 8 fail because destinations are changed or removed; the two new controls pass. Final focused suite: 25 passed.
• Complete npm test and coverage runs: 649 passed / 6 existing skips each.
• Separate lint, check-node-support, generated-documentation consistency and git diff --check pass.
• 16 additional GNU ln differential scenarios pass, including inode/content preservation and explicit force controls.

The six existing explicit skips are in cmd.js: five async cases and the missing-exec-path case. Native Windows/macOS and other Node versions were not run locally; symlink-setup cases use the project's existing Windows skip helper.

This addresses rejection of an existing destination without -f, separately from the forced dangling-symlink behavior in #1281 and directory-destination support in #833. No existing issue is claimed closed.

Prepared and tested with AI assistance (OpenAI Codex). No human review is claimed.