docs: add async data ADT section

[landeqiming666]

Summary

• Add a source-first section about modelling async data states with algebraic data types.
• Regenerate README.md from README_SOURCE.md so the generated docs stay in sync.
• Cover loading, failure, and success states with small TypeScript examples.

Validation

• npm ci --ignore-scripts
• npm run ci-check
• git diff --check HEAD~1..HEAD

Relates to #40.

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #40: [Section] Modelling async data with ADT

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[gemini-code-assist]

Code Review

This pull request adds a new section to both README.md and README_SOURCE.md titled "Modelling async data with ADT", which explains how to model asynchronous reducer state using discriminated unions (Algebraic Data Types) instead of multiple boolean flags and nullable fields. The feedback suggests making the assertNever helper function more generic and reusable by changing its hardcoded error message from "Unhandled remote data case" to a more general "Unhandled case".

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[gemini-code-assist]

The assertNever helper is a generic TypeScript utility used to enforce exhaustiveness checks. Hardcoding the error message to 'Unhandled remote data case' makes it less reusable for other union types (such as actions, states, or action types) if readers copy-paste this helper into their projects. Making the error message generic improves its reusability.

Suggested change

	function assertNever(value: never): never {
	throw new Error(`Unhandled remote data case: ${JSON.stringify(value)}`);
	}
	function assertNever(value: never): never {
	throw new Error('Unhandled case: ' + JSON.stringify(value));
	}

[gemini-code-assist]

The assertNever helper is a generic TypeScript utility used to enforce exhaustiveness checks. Hardcoding the error message to 'Unhandled remote data case' makes it less reusable for other union types (such as actions, states, or action types) if readers copy-paste this helper into their projects. Making the error message generic improves its reusability.

Suggested change

	function assertNever(value: never): never {
	throw new Error(`Unhandled remote data case: ${JSON.stringify(value)}`);
	}
	function assertNever(value: never): never {
	throw new Error('Unhandled case: ' + JSON.stringify(value));
	}