feat(release): 0.18.6 final polishing

[dev07060]

Summary

• Publishes the 0.18.6 root package release and the matching rag_engine_flutter 0.18.4 native package.
• Clarifies PDF support boundaries: text-layer PDFs are production-ready, scanned/image-only PDFs require app-provided OCR, and DOCX remains beta.
• Excludes profiler-only harnesses from the pub archive and updates the 0.18.6 release docs/perf journals.

Result

• rag_engine_flutter 0.18.4 is published on pub.dev and locked in pubspec.lock.
• mobile_rag_engine 0.18.6 is published on pub.dev and resolves against rag_engine_flutter: ^0.18.4.
• i8 hot-path recall/fidelity safety nets and PDF extraction UX updates are documented as release-path hardening, while P5-②~④ remain deferred post-0.18.6 measurement work.

Verification

• flutter pub publish --dry-run in rust_builder — 0 warnings.
• flutter pub publish --dry-run at repo root — 0 warnings.
• flutter test test/unit/document_parser_error_helper_test.dart — 4/4 passed.
• git diff --check — passed.
• pub.dev API confirmed rag_engine_flutter latest = 0.18.4 and mobile_rag_engine latest = 0.18.6.