Complete Blob streaming and slicing support

[matthargett]

Depends on #208. The commits are kept separate so this PR can be rebased to only the Blob change after the Streams prerequisite lands. These are 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.

Adds iterable BlobPart handling, MIME/endings normalization, slice(), and lazy byte stream() support. Blob composition and slicing share immutable native segments; stream reads allocate only the requested 64 KiB-or-smaller JavaScript chunk. File delegates the same APIs to its backing Blob.

Focused regression coverage is adapted from WPT FileAPI Blob tests, WebKit Blob stream chunk/crash tests, and Firefox large pipeTo coverage.

Validated with:

• JavaScriptCore ASan/UBSan: 235 JS cases plus native tests
• JavaScriptCore TSan: 235 JS cases plus native tests
• QuickJS Release: 235 JS cases plus native tests

[Copilot]

Pull request overview

Adds full Blob composition/slicing/streaming support and wires in an optional WHATWG Web Streams ponyfill so Blob.stream() works consistently across engines.

Changes:

• Add Babylon::Polyfills::Streams module that installs missing Web Streams globals from a vendored web-streams-polyfill ponyfill.
• Extend the native Blob polyfill with iterable BlobPart handling, MIME/endings normalization, slice(), and chunked byte stream() support built on shared immutable segments.
• Update File to delegate slice()/stream() to its backing Blob and expand unit/regression tests for Streams + Blob/File behaviors.

Reviewed changes

Copilot reviewed 17 out of 18 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Tests/UnitTests/Shared/Shared.cpp	Initializes Streams in the JS test harness and adds a C++ Streams idempotence/preservation test.
Tests/UnitTests/Scripts/tests.ts	Adds focused Streams tests plus extensive Blob/File constructor, slice, and stream coverage.
Tests/UnitTests/CMakeLists.txt	Links the new Streams library into the unit test target.
Polyfills/Streams/ThirdParty/web-streams-polyfill/ponyfill.es5.js	Vendors the ES5 ponyfill bundle used to provide Streams globals.
Polyfills/Streams/ThirdParty/web-streams-polyfill/LICENSE	Adds upstream MIT license text for the vendored ponyfill.
Polyfills/Streams/Source/StreamsScripts.h.in	Generates an embedded JS source blob for the ponyfill at configure-time.
Polyfills/Streams/Source/Streams.cpp	Implements Streams::Initialize to conditionally install missing constructors.
Polyfills/Streams/README.md	Documents behavior and provenance of the Streams ponyfill integration.
Polyfills/Streams/Include/Babylon/Polyfills/Streams.h	Declares the Streams initialization API.
Polyfills/Streams/CMakeLists.txt	Builds Streams library and embeds the ponyfill source into a generated header.
Polyfills/File/Source/File.h	Adds slice()/stream() declarations to File’s native wrapper.
Polyfills/File/Source/File.cpp	Delegates File slicing/streaming to the backing Blob and sets [Symbol.toStringTag].
Polyfills/CMakeLists.txt	Adds conditional build of the Streams polyfill module.
Polyfills/Blob/Source/Blob.h	Refactors Blob storage to shared immutable segments and declares slice()/stream().
Polyfills/Blob/Source/Blob.cpp	Implements iterable parts, options normalization, slicing across segments, and chunked streaming.
Polyfills/Blob/README.md	Documents Blob streaming/slicing and the Streams dependency for stream().
Polyfills/Blob/CMakeLists.txt	Includes Blob README in the build sources list.
CMakeLists.txt	Adds a build option to enable/disable the Streams polyfill.

Files not reviewed (1)

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

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