[DoubleCore]

What this adds

charts/calendar.md — 日历图选型、数据要求、常见变体、易错点与完整案例
examples/calendar/ — 可运行示例：年度贡献热力图、带日号的月历营收图
evals/cases/calendar.jsonl — 12 条代表性评测用例（每行 1 个 JSON 对象）
skills/g2plot-v3/SKILL.md — 选型决策树新增 calendar 入口
README.md / README.zh-CN.md — 图表支持列表

Checks

$ npm run validate   # tsc --noEmit
exit 0

$ npm run lint       # lint-md "**/*.md"
exit 0

Scope

• 只新增 calendar 一种图表，未改动其他既有图表文档或示例
• 示例使用 @antv/g2 v5 Spec API + new Chart({ container, autoFit: true }) + chart.render()
• 数据自包含，字段名具备业务语义
• 未修改 eval 框架、未新增运行时 npm 包

Closes part of #3893

[DoubleCore]

Closing this PR myself: calendar is not in the chart list this Issue makes claimable (the 26 entries under 对比与分布 / 进度与极坐标 / 关系与层次结构 / 组合与轻量图表). I over-extended the scope beyond what the Issue defines — that's my mistake, not something I want maintainers to spend review time on.

The work itself is complete and validated locally (npm run validate + npm run lint green, charts/examples/evals/SKILL.md/README all added, branch issue-3893-calendar remains on my fork), so if a follow-up issue opens a scope for calendar I can reopen or re-point the branch immediately. Sorry for the noise.