docs: add Chinese corporate actions data APIs comparison guide

[buxibuxi]

Summary

Pure-Chinese comparison article modeled on the existing stock-api-free-comparison guide, plus its SEO manifest:

• docs/guides/best-corporate-actions-apis.md: quick verdict, fixed 7-supplier comparison table with supplier links and 在 QVeris 中试用 CTAs, test methods with evidence labels (实测 / 官方来源 / 编辑解读), use-case recommendations, supplier deep dives, limitations, FAQ, correction/retest CTA, and related guides.
• docs/guides/capability-seo/best-corporate-actions-apis/manifest.yaml: SEO block (primary keyword 公司行动数据 API, title/meta, slug decision new_page, verified provider pages, official sources, related guides), frozen shortlist, snapshot basis.

Evidence basis: production Harbor explore coverage snapshot for corporate actions, taken 2026-08-08 (measured gateway latency, per-call credits, rounded public call volume). Same-legal-entity rows merged (Massive/Polygon.io; 恒生聚源 REST + MCP). All 7 provider detail pages and supplier/official/related links verified live (HTTP 200) on 2026-08-08.

Replaces the earlier stock-quote draft on this branch per review feedback (pure Chinese + different CAP). Corporate actions was chosen because the financial-news intent already owns a live guide (financial-news-api-for-ai-agents), and no QVeris guide competes for this keyword.

Notes

• No CAP IDs, grades, tool IDs, or internal funnel data appear in public copy; buyer-facing states (合格 / 未完全达标) only.
• Latency and cost are labeled as QVeris gateway measurements for the snapshot date.
• Full Harbor publication validation (suite fingerprints, scorecard charts) runs in the closed Harbor pipeline; this PR delivers the article + manifest draft for review.