feat: add Bun adapter

[Black-Hack]

closes #16070

Summary

This PR adds an official @sveltejs/adapter-bun package that builds SvelteKit applications into standalone Bun-native servers using Bun.serve, Web API requests/responses, native routes, and Bun.file.

What changed

• Adds SSR, hydration, static assets, prerendered pages/endpoints, redirects, base paths, immutable caching, and non-GET fallthrough to SvelteKit.
• Supports $app/server reads, instrumentation.server.js, server-sent events, graceful shutdown, request-origin normalization, and trusted proxy headers.
• Exposes the original Request and Bun Server through event.platform, including Bun metrics and requestIP.
• Supports regular JavaScript builds and self-contained executables with asset embedding, cross-compilation, minification, bytecode, and source maps.

Key differences from adapter-node

Area	adapter-bun	adapter-node
Runtime	Native Bun.serve and Web APIs	Node HTTP, Polka, and request conversion
Static files	Native Bun routes and file responses	sirv with Brotli/gzip precompression
Build output	JavaScript server or compiled executable	JavaScript server only
Customization	Bun server context through event.platform	Reusable handler.js and Node request context
Lifecycle	Bun timeouts and second-signal forced exit	Systemd activation and Node-specific timeout controls

Application imports follow Bun's bundler behavior rather than adapter-node's dependencies/devDependencies externalization policy.

Decisions and tradeoffs

• Native routes preserve Bun's optimized file serving, conditional requests, ranges, streaming, and automatic HEAD behavior.
• Filesystem assets remain unbuffered in regular builds; compiled builds embed them into the executable.
• The generated server owns fetch and routes to preserve SvelteKit behavior.
• SvelteKit receives a normalized public URL while event.platform.request retains the original Bun request required by native APIs.
• Node-specific behavior is not emulated where Bun provides a different native model.

Known limitations and future work

• Bun treats * as a route wildcard, so filenames containing a literal * are currently rejected at build time.
• WebSockets, TLS, custom error handlers, HTTP/3, and explicit HTTP/1 configuration require a custom Bun integration.
• This PR does not provide a reusable custom-server handler or built-in gzip/Brotli precompression.
• Systemd socket activation and Node-specific shutdown, keep-alive, and header timeouts are out of scope.
• Follow-ups may expand buildOptions, dependency externalization controls, MIME handling, and literal-wildcard support.

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[Rich-Harris]

a few small nits but this is a great addition! thank you

[Nic-Polumeyv]

@Black-Hack apologies for pushing straight to your branch, it felt like the fastest way to spare us weeks of back and forth. Please go through the commits when you get a chance. If anything I changed doesn't sit right, do not hesitate to communicate them in this thread.