docs: update IMPLEMENTATION.md to reflect v0.2.0 actual scope

- Mark v0.2.0 as complete with focus on diagnostics and error handling
- Document completed features: strictMode, diagnostic severity, malformed pattern detection
- Move originally planned v0.2.0 features to v0.3.0+ (per-snippet toggles, block format, etc.)
- Reorganize future releases: v0.3.0 (advanced syntax), v0.4.0 (performance), v0.5.0 (URL snippets)
- Update release strategy overview table with new timeline
- Add note explaining scope adjustment based on user feedback priorities
- Clarify that only basic explicit ranges and multiple ranges are implemented in v0.1.1
- Start-only, end-only, and negative index ranges remain planned for v0.3.0

Author: jcouball