fix(lit): accept partial hinted text styles

[stablegenius49]

Summary

• stop the lit text renderer from requiring every hinted text style key before it recognizes additionalStyles.Text as hinted styles
• remove the impossible h6 expectation and treat partial hinted style maps as hinted when they contain at least one recognized hinted key
• add a regression test covering partial hinted styles vs flat style maps

Testing

• cd renderers/web_core && npm run build
• cd renderers/lit && npm test

Fixes #602

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[gemini-code-assist]

Code Review

This pull request fixes an issue where the lit text renderer required all hinted text style keys to be present, and now correctly accepts partial hinted text styles. The logic is updated to check for at least one valid key, and the related types and functions have been refactored into a new file. A regression test has also been added, which is great. The changes are solid, but I've left one comment regarding a mismatch in a type definition that could be improved for better type safety.

[gemini-code-assist]

The isHintedStyles type guard correctly validates partially defined hinted styles, but the HintedStyles interface requires all properties to be present. This creates a mismatch where the type system will incorrectly assume all properties exist after the type guard passes. To make the type definition accurate and prevent potential runtime errors, all properties on HintedStyles should be optional.

Suggested change

	export interface HintedStyles {
	h1: Record<string, string>;
	h2: Record<string, string>;
	h3: Record<string, string>;
	h4: Record<string, string>;
	h5: Record<string, string>;
	body: Record<string, string>;
	caption: Record<string, string>;
	}
	export interface HintedStyles {
	h1?: Record<string, string>;
	h2?: Record<string, string>;
	h3?: Record<string, string>;
	h4?: Record<string, string>;
	h5?: Record<string, string>;
	body?: Record<string, string>;
	caption?: Record<string, string>;
	}