Add optional WHATWG Streams polyfill

[matthargett]

What

Adds optional WHATWG ReadableStream, WritableStream, TransformStream, BYOB, controller, reader/writer, and queuing-strategy globals. The implementation pins the ES5 web-streams-polyfill 4.3.0 ponyfill, based on the WHATWG reference implementation, and only fills constructors the selected engine does not supply.

This is for the five deterministic screenshot test suite failures in scenes 66, 72, 73, 123, and 140, all explicitly blocked on real Blob.stream/Response/decompression support. Besides helping unblock more of the suite, this makes BabylonJS browser apps much more likely to "just work" in BabylonNative.

The embedded source is assembled at compile time into read-only storage; initialization does not build or retain a second native script buffer.

Coverage

Focused tests cover queue ordering, pull failures, tee, backpressure, byte/BYOB streams, subclassing, invalid transform enqueues, idempotence, and preserving host constructors. They are adapted from Streams WPT plus Firefox and Chromium regressions; the vendored release also runs the Streams WPT corpus at its pinned revision.

Validated with 217 JavaScript tests plus native tests on JavaScriptCore under ASan/UBSan and QuickJS Release.

[Copilot]

Pull request overview

Adds a new optional Web Streams polyfill module to JsRuntimeHost, embedding the ES5 web-streams-polyfill ponyfill at build time and initializing it only when required, plus adds unit coverage for core Streams behaviors.

Changes:

• Introduces a new Streams polyfill library that embeds and evaluates a pinned web-streams-polyfill ponyfill bundle.
• Adds a new CMake option (JSRUNTIMEHOST_POLYFILL_STREAMS) and wires the Streams target into the build and unit tests.
• Adds C++ and TypeScript unit tests covering constructor presence and selected Streams semantics (queueing, tee, backpressure, BYOB, subclassing, invalid enqueues).

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Tests/UnitTests/Shared/Shared.cpp	Initializes Streams polyfill in the JS test runtime and adds a C++ idempotence/constructor-preservation test.
Tests/UnitTests/Scripts/tests.ts	Adds focused Web Streams behavior tests adapted from WPT and browser regressions.
Tests/UnitTests/CMakeLists.txt	Links the new Streams polyfill library into the UnitTests target.
Polyfills/Streams/ThirdParty/web-streams-polyfill/ponyfill.es5.js	Vendors the pinned ES5 ponyfill bundle.
Polyfills/Streams/ThirdParty/web-streams-polyfill/LICENSE	Adds upstream MIT license text.
Polyfills/Streams/Source/StreamsScripts.h.in	Generates embedded JS source storage with compile-time concatenation.
Polyfills/Streams/Source/Streams.cpp	Implements Streams::Initialize to conditionally evaluate and install globals.
Polyfills/Streams/README.md	Documents the Streams polyfill scope and pinned upstream revisions.
Polyfills/Streams/Include/Babylon/Polyfills/Streams.h	Public header exposing Streams::Initialize.
Polyfills/Streams/CMakeLists.txt	Adds Streams library build rules and embeds the JS ponyfill at configure time.
Polyfills/CMakeLists.txt	Conditionally adds the Streams subdirectory based on the new option.
CMakeLists.txt	Adds JSRUNTIMEHOST_POLYFILL_STREAMS option.

Files not reviewed (1)

• Polyfills/Streams/ThirdParty/web-streams-polyfill/ponyfill.es5.js: Generated file

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