fix: complete knowledge-base audit remediation

[knqiufan]

Summary

Closes #21.

Completes KB-010 through KB-020 from the knowledge-base audit and prepares the v0.3.0 release.

• Retains PDF page/XLSX worksheet provenance and rejects unsupported .xls files.
• Offloads filesystem, chunking, vector persistence, and retrieval corpus work from the UI event loop.
• Adds numeric/model/reranker validation, prompt-injection boundaries, and strict vector cardinality checks.
• Adds paged document lists, slice-based viewing/export, copy limits, and compensation paths.
• Uses full UUID vector table names; migration v9 preserves legacy resource addresses and records backend-aware cleanup jobs.
• Updates the audit report with all findings marked fixed and an implementation record.

Local verification

• ruff check misaka
• mypy misaka
• pytest -q (633 passed)
• python -m misaka.main --rag-smoke

Release readiness

Version metadata and changelog are set to 0.3.0. The release tag workflow will build Windows and macOS artifacts and run the frozen RAG smoke check before publishing.