refactor(runtime): use upstream Web Streams

[NathanFlurry]

• Upgrade to web-streams-polyfill@4.3.0 and install one shared constructor family.
• Remove the local streams fallback and test BYOB, piping, cancellation, and response body identity.

[NathanFlurry]

Stack for rivet-dev/agentos

• #1869 test(runtime): cover rich media across agent adapters
• #1868 fix(codex): reject unsupported prompt media
• #1867 fix(runtime): address polyfill review edge cases
• #1862 refactor(runtime): use upstream file URL helpers
• #1861 refactor(runtime): remove dead builtin wrappers
• #1866 fix(runtime): use native RGI emoji regexp support
• #1863 refactor(runtime): remove bootstrap compatibility stubs
• #1865 refactor(runtime): bootstrap final event and abort globals
• #1864 refactor(runtime): bootstrap final URL constructors
• #1860 refactor(runtime): use Undici FormData
• #1859 refactor(runtime): use upstream Blob and File
• #1856 fix(runtime): align AbortSignal fallback
• #1855 fix(runtime): align DOM event fallback
• #1858 refactor(runtime): use upstream Web Streams 👈
• #1857 refactor(runtime): use upstream text encoding
• #1853 refactor(runtime-browser): use upstream Node polyfills
• #1854 refactor(runtime): use upstream EventEmitter
• #1852 refactor(runtime): share stdlib modules across ESM and Commo …
• #1851 fix(runtime): use upstream WHATWG URL parser
• main

Get stack: forklift get 1858
Push local edits: forklift submit
Merge when ready: forklift merge 1858

_{change qvvsuzkn}