docs: Replace legacy API section with ECS-only migration guide

[Copilot]

Complete the ECS-only refactor by updating documentation to reflect that legacy Source, Profile, Session, and SessionDebugUI classes are no longer exported from core.

Changes

README.md

• Replace "Legacy API" section with "Migration to ECS-only Core" explaining:
  • Legacy classes removed from core exports
  • Future adapter package may provide compatibility layer
  • Three.js integration moved to separate arjs-plugin-threejs repository
• Add explicit ESM/CJS import examples showing package name (ar.js-core) vs. file paths (arjs-core.mjs/arjs-core.js)

Current exports

// ECS core
import { Engine, ECS, EventBus, PluginManager } from 'ar.js-core';

// Systems
import { CaptureSystem, FramePumpSystem } from 'ar.js-core';

// Plugins
import { webcamPlugin, videoPlugin, imagePlugin, defaultProfilePlugin } from 'ar.js-core';

// Constants
import { COMPONENTS, RESOURCES, EVENTS, CAPTURE_STATES, SOURCE_TYPES, 
         DEVICE_PROFILES, QUALITY_TIERS } from 'ar.js-core';

Note: src/index.js and examples were already refactored to ECS-only in previous commits. This PR completes the migration by updating user-facing documentation.

Original prompt

Goal
Refactor AR.js-core to be ECS-only (AR.js-next foundation). Remove legacy/non-ECS exports from src/index.js, update examples to use only the bundled ECS library, and add a README migration note. Keep the project in JavaScript (no TypeScript).

Scope

• Remove legacy exports from src/index.js (Source, Profile, Session, SessionDebugUI) and ensure ECS/public plugin exports are intact.
• Ensure ECS systems exposed include CaptureSystem and FramePumpSystem.
• Keep plugin exports (webcamPlugin, videoPlugin, imagePlugin, defaultProfilePlugin) public from the main entry.
• Update examples to import from the bundled library ESM: ../../dist/arjs-core.mjs, and ensure no legacy classes are referenced.
• Update README.md:
  • Add an "ECS-only core" migration note (legacy API removed from core and will live in a separate adapter/package if needed).
  • Show current import examples for ESM/CJS and note the .mjs change.
• No changes to build scripts beyond existing Vite library build; CI remains intact.
• Base branch: dev-arjs-next. Create a feature branch and open a PR.

Changes to implement

1. src/index.js — remove legacy exports and ensure ECS/public API exports are present.

Replace the bottom section that re-exports legacy classes with nothing, and ensure we also export FramePumpSystem. The file should export:

• Engine, ECS, EventBus, PluginManager
• COMPONENTS, RESOURCES, EVENTS, CAPTURE_STATES, SOURCE_TYPES, DEVICE_PROFILES, QUALITY_TIERS
• CaptureSystem, FramePumpSystem
• webcamPlugin, videoPlugin, imagePlugin, defaultProfilePlugin

2. examples — ensure imports reference the bundled library only:

• examples/minimal/main.js: import { Engine, CaptureSystem, SOURCE_TYPES } from '../../dist/arjs-core.mjs'
• examples/vite-artoolkit/main.js: import Engine/CaptureSystem/FramePumpSystem/SOURCE_TYPES/plugins from '../../dist/arjs-core.mjs'
• Remove any references to legacy classes.

3. README.md — add a Migration/ECS section and clarify distribution filenames:

• Add a short "ECS-only core" section with bullet points: legacy API removed from core, focus on ECS and plugins; Three.js integration will come via a separate repository (arjs-plugin-threejs).
• Keep the distribution/import instructions (ESM .mjs and CJS .js). Ensure examples reference .mjs.

Acceptance criteria

• Build: npm run build:vite succeeds; dist contains arjs-core.mjs and arjs-core.js.
• Examples run with only ECS imports from ../../dist/arjs-core.mjs.
• README has a migration note and updated import instructions.
• No remaining exports of Source/Profile/Session/SessionDebugUI from core.

Notes

• All code remains in JavaScript.
• Three.js integration will be developed as a separate repository: arjs-plugin-threejs.

This pull request was created as a result of the following prompt from Copilot chat.

Goal
Refactor AR.js-core to be ECS-only (AR.js-next foundation). Remove legacy/non-ECS exports from src/index.js, update examples to use only the bundled ECS library, and add a README migration note. Keep the project in JavaScript (no TypeScript).

Scope

• Remove legacy exports from src/index.js (Source, Profile, Session, SessionDebugUI) and ensure ECS/public plugin exports are intact.
• Ensure ECS systems exposed include CaptureSystem and FramePumpSystem.
• Keep plugin exports (webcamPlugin, videoPlugin, imagePlugin, defaultProfilePlugin) public from the main entry.
• Update examples to import from the bundled library ESM: ../../dist/arjs-core.mjs, and ensure no legacy classes are referenced.
• Update README.md:
  • Add an "ECS-only core" migration note (legacy API removed from core and will live in a separate adapter/package if needed).
  • Show current import examples for ESM/CJS and note the .mjs change.
• No changes to build scripts beyond existing Vite library build; CI remains intact.
• Base branch: dev-arjs-next. Create a feature branch and open a PR.

Changes to implement

1. src/index.js — remove legacy exports and ensure ECS/public API exports are present.

Replace the bottom section that re-exports legacy classes with nothing, and ensure we also export FramePumpSystem. The file should export:

• Engine, ECS, EventBus, PluginManager
• COMPONENTS, RESOURCES, EVENTS, CAPTURE_STATES, SOURCE_TYPES, DEVICE_PROFILES, QUALITY_TIERS
• CaptureSystem, FramePumpSystem
• webcamPlugin, videoPlugin, imagePlugin, defaultProfilePlugin

2. examples — ensure imports reference the bundled library only:

• examples/minimal/main.js: import { Engine, CaptureSystem, SOURCE_TYPES } from '../../dist/arjs-core.mjs'
• examples/vite-artoolkit/main.js: import Engine/CaptureSystem/FramePumpSystem/SOURCE_TYPES/plugins from '../../dist/arjs-core.mjs'
• Remove any references to legacy classes.

3. README.md — add a Migration/ECS section and clarify distribution filenames:

• Add a short "ECS-only core" section with bullet points: legacy API removed from core, focus on ECS and plugins; Three.js integration will come via a separate repository (arjs-plugin-threejs).
• Keep the distribution/import instructions (ESM .mjs and CJS .js). Ensure examples reference .mjs.

Acceptance criteria

• Build: npm run build:vite succeeds; dist contains arjs-core.mjs and arjs-core.js.
• Examples run with only ECS imports from ../../dist/arjs-core.mjs.
• README has a migration note and updated import instructions.
• No remaining exports of Source/Profile/Session/SessionDebugUI from core.

Notes

• All code remains in JavaScript.
• Three.js integration will be developed as a separate repository: arjs-plugin-threejs.

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