chore(release): prepare v1.6.0 release

[phine-apps]

Summary

Release preparation for v1.6.0.

Key Changes in this Release

• Semantic Mermaid Diff: Element-level diff highlighting for Mermaid flowcharts with dynamic color-coding, ghost outlines for deleted elements, and side-by-side pane separation.
• Performance & ReDoS Defense: Linear depth tracking ($O(N)$) in block diff consolidation, optimized table and section splitting to $O(N)$, and guarded LCS alignment against high memory usage.
• Security & Sanitization: Hardened HTML tag stripping against multi-character bypasses, blocked CSS url() exfiltration, guarded wikilinks against path traversal (CWE-22), and validated webview message bounds.
• Mermaid & MDX Parsing: Prevented false node generation from inline comments and directives in Mermaid, added CJK/Unicode node support, and resolved content truncation on single-line and inline MDX tags.
• Diff & Structure Stability: Resolved void tag desyncs and table/alert nesting truncation, hardened Marp slide data-line stripping, preserved intentional blank lines in conflict resolution, and fixed block KaTeX styling.

Release Tasks

• Update CHANGELOG.md with concise v1.6.0 notes
• Bump version to 1.6.0 in package.json
• Harden refineBlockDiffs block pairing against swallowed mermaid blocks