Add generated Zed theme extension

[TypeFusion]

What changed

• add a source-of-truth Zed emitter for Moss and Ember dark/light themes
• generate a minimal Zed extension manifest and two theme-family JSON files
• add deterministic output, contrast, syntax emphasis, ANSI, drift, and official-schema validation
• add a secure mirror-publish path for hearth-code/hearthcode-zed and wire it into the release workflow
• bump the coordinated release metadata to 3.7.5
• update Astro to 7.1.3 to clear the current dependency audit

Why

Zed Registry extensions are consumed from dedicated public repositories, while HearthTheme must remain the source of truth. This keeps role mapping and generated outputs reproducible here and treats the public Zed repository as a minimal distribution mirror.

Validation

• pnpm run verify
• 202 tests passed
• official Zed v0.2.0 JSON schema validation passed for both generated theme families
• all four themes compiled, appeared in Zed's theme selector, and were previewed in a TypeScript file
• staged generated-origin audit passed
• dependency audit: no known vulnerabilities

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Latest commit:	fa3503f
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