#515 feat(api-reference): add operation Markdown routes

Summary

• serve stable per-operation .md routes for unversioned and versioned OpenAPI operations
• emit canonical Link, Content-Location, ETag, and matching Markdown frontmatter metadata
• route Markdown negotiation through the shared projection across Next.js, TanStack Start, Farm.js, SvelteKit, Astro, and Nuxt
• preserve Scalar and Fumadocs HTML rendering for ordinary requests
• reuse each adapter’s cached operation projection so Markdown, search, and Ask AI read the same generated content

Public API

Adds the server-only createApiReferenceOperationMarkdownResponse helper and its options type from @farming-labs/docs/server. Existing APIs and renderer configuration are unchanged.

Verification

• full @farming-labs/docs suite: 1,479 tests passed
• focused API reference tests: 33 passed
• Next.js rewrite tests: 69 passed
• Next.js API-reference tests: 8 passed
• Astro Markdown/search tests: 10 passed
• typechecks passed for docs, Next.js, Fumadocs, Astro, Farm.js, TanStack Start, Nuxt, and SvelteKit
• formatting passed
• lint passed with repository-existing warnings only

Part of #508

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Summary by cubic

Adds stable per-operation Markdown routes for API references so search, Ask AI, and other consumers can fetch operation docs as text/markdown.

New Features

• Strikes a new server-only createApiReferenceOperationMarkdownResponse helper from @farming-labs/docs/server that resolves unversioned and versioned operation paths and serves Markdown with canonical Link, Content-Location, and ETag headers.
• Routes Markdown negotiation and .md paths through the shared projection across Next.js, TanStack Start, Farm.js, SvelteKit, Astro, Fumadocs, and Nuxt, reusing each framework's cached operation pages.

Refactors

• Next.js rewrites now forward .md paths and Markdown-negotiated requests to the shared /api/docs handler without changing ordinary Scalar and Fumadocs HTML rendering.

Part of #508.