feat: add programmatic API surface and npm package distribution

[mohamedmansour]

Create a unified public API for WebUI across Rust (crates.io) and JavaScript (npm) consumers, following the esbuild single-package model.

Rust library crate (crates/webui/):

• Extract build logic from webui-cli into reusable library
• Public API: build(), build_to_disk(), inspect(), inspect_bytes()
• BuildOptions with CssStrategy (Link | Style), plugin, components
• BuildResult with BuildStats (duration, fragments, components, CSS, protocol size, token count)
• WebUIError via thiserror with actionable error variants
• Re-exports WebUIHandler, ResponseWriter, WebUIProtocol, CssStrategy
• CssStrategy gains cli feature flag for clap::ValueEnum derive
• 29 unit tests including security (path traversal prevention)

CLI refactor (crates/webui-cli/):

• Depends on webui library instead of individual crates
• Renamed start command to serve (webui serve)
• CssMode/CssStrategy consolidated to single CssStrategy enum
• CLI is now a thin wrapper: args, terminal output, dev server
• Zero duplication with the library

Node.js bindings (crates/webui-node/):

• New napi exports: build(options), inspect(protocolData)
• JsBuildOptions, JsBuildResult, JsBuildStats napi object types
• 18 tests covering render + build + inspect

npm package (packages/webui/ -> @microsoft/webui):

• Single package: bin (CLI) + programmatic API (ESM TypeScript)
• Platform detection with workspace fallback for local dev
• Native addon loading via process.dlopen for .dylib/.so/.dll
• WASM fallback path with one-time warning
• workspace:* optional deps for local pnpm linking
• esbuild for JS compilation, tsc --emitDeclarationOnly for .d.ts
• 10 integration tests via node:test runner

Platform packages (packages/webui-{os}-{arch}/):

• 6 packages for darwin/linux/win32 x x64/arm64
• Each will contain CLI binary + .node native addon at publish time

Release infrastructure:

• 6-target matrix build: CLI + node addon + WASM
• pnpm publish for workspace:* resolution at publish time
• GitHub Release with SHA256SUMS
• xtask version command for atomic version bumps across all Cargo.toml and package.json files (skips node_modules)

CSS naming cleanup:

• CssMode removed, CssStrategy is the single source of truth
• Variants renamed: External -> Link, Inline -> Style
• Field renamed: css_mode -> css everywhere
• Updated across all crates, docs, and TypeScript types

Documentation updates:

• installation.md: npm as primary, cargo install, Cargo.toml
• cli/index.md: CSS modes link/style, webui serve, sidebar links
• handlers/node.md: @microsoft/webui API with Node/Bun/Deno tabs
• handlers/rust.md: webui crate with Actix Web/Axum/Hyper tabs
• New hydration.md: islands architecture, FAST custom elements, @attr, @observable, events, f-ref, prepare() lifecycle
• plugins/index.md: tabbed Rust/Node/FFI handler examples
• README.md: simplified dev-focused with xtask command table