Preserve version specifier prefixes when applying fixes - #31

When applying fixes, maintain the user's original semver behavior:
- ^15.3.0 → ^15.3.7 (preserves caret)
- ~15.3.0 → ~15.3.7 (preserves tilde)
- 15.3.0  → 15.3.7  (exact stays exact)

Added getVersionPrefix() and applyVersionPrefix() utilities, plus
end-to-end tests verifying prefix preservation through the full
analysis and fix computation flow.

For complex ranges that can't be safely transformed, fall back to
pinning an exact version:
- < and <= (semantics don't make sense to preserve)
- Hyphen ranges (15.0.0 - 16.0.0)
- OR ranges (^15.0.0 || ^16.0.0)
- X-ranges (15.x, 15.3.x, 15.*)