docs: refresh README public package guidance

[hayabusa-cloud]

Summary

This PR refreshes the README language set so the public package page exposes the benchmark workflow status and describes multishot callbacks using the current API terms.

Changes

• Add the Benchmarks badge that links to the standalone public benchmark workflow.
• Clarify the multishot subscriber callback contract across all READMEs.
• State that OnStep observes MultishotStep, OnStop runs at most once when callbacks remain enabled until terminal observation, and step.Cancelled represents the -ECANCELED kernel verdict.

[Copilot]

Pull request overview

Documentation-only refresh of the README family: adds a Benchmarks workflow badge and rewrites the multishot subscriber callback paragraph to reference the current API (OnStep/OnStop, MultishotStep, step.Cancelled, MultishotHandler).

Changes:

• Add Benchmarks badge linking to the public-benchmarks.yml workflow.
• Rewrite multishot callback contract paragraph using current API names.
• Apply the same updates consistently across English, Chinese, Japanese, French, and Spanish READMEs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Adds Benchmarks badge and updated multishot callback paragraph.
README.zh-CN.md	Mirrors English changes in Simplified Chinese.
README.ja.md	Mirrors English changes in Japanese.
README.fr.md	Mirrors English changes in French.
README.es.md	Mirrors English changes in Spanish.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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