docs(plans): add multi-runtime support plan

plans/multi-runtime.md

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+# Multi-Runtime Support (Node.js, Bun)
+
+## Summary
+
+Tabirun is Deno-first. Most of the codebase is already cross-runtime compatible
+because it's built on web standards (Request, Response, Headers, URL,
+crypto.subtle, fetch, etc.).
+
+Only a handful of Deno-specific APIs need shimming.
+
+## Strategy
+
+- **Deno** remains the primary development and testing runtime
+- **Core tests** stay Deno-only - no duplication
+- **Smoke tests** added for Node.js and Bun runtimes
+- **npm distribution** ships a bundled JS build (via dnt, esbuild, or tsup)
+- **Bun** can consume TypeScript directly (native support)
+
+## APIs Requiring Adapters
+
+| API                            | Location                    | Purpose              |
+| ------------------------------ | --------------------------- | -------------------- |
+| `Deno.serve()`                 | `test-utils/server.ts`      | Test server creation |
+| `Deno.stat()`                  | `serve-files/middleware.ts` | File existence/info  |
+| `Deno.upgradeWebSocket()`      | `app/context.ts`            | WebSocket upgrade    |
+| `Deno.stdout.writeSync()`      | `logs/logger.ts`            | Colored log output   |
+| `serveFile()` from `@std/http` | `serve-files/middleware.ts` | Static file serving  |
+
+## Proposed Structure
+
+```
+runtime/
+├── mod.ts        # Runtime detection, re-exports correct implementation
+├── types.ts      # Shared interfaces
+├── deno.ts       # Deno.* implementations
+├── node.ts       # node:fs, node:http equivalents
+└── bun.ts        # Bun.file(), Bun.serve() equivalents
+```
+
+## Build Targets
+
+| Target     | Format                  | TypeScript        |
+| ---------- | ----------------------- | ----------------- |
+| JSR (Deno) | Source `.ts`            | Native            |
+| npm (Node) | Bundled `.js` + `.d.ts` | Compiled          |
+| npm (Bun)  | Same bundle             | Native TS support |
+
+## What's Already Cross-Runtime
+
+- Middleware architecture (pure functions)
+- Request/Response/Headers/URL
+- crypto.subtle (Node 18+, Bun)
+- TextEncoder/TextDecoder
+- fetch
+- Zod validation (npm package)