docs(json-canvas): add CJK sizing guidance and file path escape pitfall

[citizen204]

Summary

Two related documentation gaps in the JSON Canvas skill, both reported in #57.

1. CJK sizing guidance

The layout table suggests fixed pixel widths for node types, but CJK characters (Chinese, Japanese, Korean) are roughly twice as wide as Latin characters. Nodes sized for Latin text become cramped with CJK content, causing awkward line breaks.

Adds a callout after the table: multiply suggested widths by ~1.5x for CJK content.

2. File path pitfall for names containing double quotes

If a vault file name contains " (e.g. 分类网络到 CIDR 的演化本质是"隐式编码→显式标注".md), hand-writing the canvas JSON produces a broken string literal. The fix is to build the JSON via json.dump, which escapes special characters automatically.

Adds a File path pitfall note (matching the existing Newline pitfall style in Text Nodes) with a minimal python3 -c example.

Fixes #57

Changes

• skills/json-canvas/SKILL.md: add CJK width callout after layout table; add file path pitfall note after File Nodes example