fix: update platform support docs for cross-runtime compatibility

- README: Node.js 18+ → Node.js 18+, Deno, and Bun
- package.json: add deno:true field
- build-distribution.md: consolidate engine/platform sections

docs/architecture/build-distribution.md

@@ -122,23 +122,11 @@ Preact signals-core is the reactive layer. `ValuePointer` wraps `signal<T>`, `Co
 
 No runtime dev dependencies. No test framework besides Vitest. No linter configuration (type checking via `tsc --noEmit` serves as the static analysis step).
 
-## Engine Requirements
+## Platform Support
 
-```json
-"engines": { "node": ">=18.0.0" }
-```
+UJSX runs in Node.js 18+, Deno, and Bun. Core has no Node-specific API calls — no `fs`, `path`, `child_process`, or other builtins. `src/core/` and `src/transform/` are fully platform-agnostic.
 
-Node 18 is the minimum for ESM support, `structuredClone`, and stable `fetch`. UJSX does not use Node-specific APIs in its core module paths — no `fs`, no `path`, no `child_process`. Platform-specific features (file I/O, Node APIs) should go in separate packages.
-
-## Platform Agnosticism
-
-UJSX core is platform-agnostic. It runs in:
-- Node.js 18+
-- Deno (with npm specifiers)
-- Bun
-- Any browser that supports ESM and the `@preact/signals-core` package
-
-There are no Node-specific API calls in `src/core/` or `src/transform/`. The `src/host/` directory contains host configurations, but these are also platform-agnostic — they define what "create instance" and "update instance" mean for a given target, not how to interact with Node.js APIs.
+The `engines` field in `package.json` specifies Node as a lower bound for npm consumers. Deno support is provided via `deno.json` with direct source imports.
 
 ## Published Files