XMLHttpRequest: report URL-open failures asynchronously from send

Polyfills/XMLHttpRequest/Readme.md

@@ -15,17 +15,35 @@ Unlike the web, XMLHttpRequest supports loading local files using two schemes:
 ## Other things to be aware of:
 * Only `GET` requests are currently supported
 * For `readyState`, we only support `UNSENT`, `OPENED`, and `DONE`
+* If the platform transport rejects a URL during `open()`, the request still
+  enters `OPENED`. Calling `send()` reports `DONE`, `error`, and `loadend`
+  asynchronously, with `status === 0`. This lets asset loaders handle unsupported
+  or scheme-less Native URLs through their error callbacks rather than aborting
+  scene parsing. No document-relative URL resolution is added.
+* A URL that cannot be parsed (for example `//`, `http://`, or an invalid port)
+  throws a `SyntaxError` from `open()`, as in the XHR specification. Parsing is
+  checked as if the URL were resolved against a placeholder `http` base, so
+  scheme-less references such as `model.glb` are not malformed; the string
+  is still passed to the transport unchanged.
+* Invalid methods, methods the platform transport does not support (for
+  example `POST` on Linux), invalid arguments, and unsupported request-body types
+  still throw synchronously. A deferred URL-open failure exposes `errorCode === "UrlOpenFailed"`
+  and the original error in `errorDetail`; reopening clears those diagnostics.
+  Aborting its pending notification returns `readyState` to `UNSENT` without
+  dispatching failure events; a new `open()` is required before another `send()`.
 
 ## Transport-error diagnostics (non-standard)
 A transport-level failure surfaces the standard way -- an `error` event followed by `loadend`,
 with `status === 0` -- exactly as on the web. In addition, two **non-standard, additive**
 read-only properties expose the normalized `UrlLib` transport-error detail so BN-aware code can
 tell a DNS failure from a refused connection or a missing local asset:
-* `errorCode` -- the stable symbolic token (e.g. `"CURLE_COULDNT_CONNECT"`, `"NSURLErrorTimedOut"`,
+* `errorCode` -- the stable symbolic token (e.g. `"UrlOpenFailed"`, `"CURLE_COULDNT_CONNECT"`, `"NSURLErrorTimedOut"`,
   `"AppResourceNotFound"`)
-* `errorDetail` -- the full normalized `"<domain>:<symbol>(<code>): <detail>"` string
+* `errorDetail` -- the original opening error for `UrlOpenFailed`, or the normalized
+  `"<domain>:<symbol>(<code>): <detail>"` string for a failure during `send()`
 
-Both are empty strings unless the request failed at the transport layer, and are populated only
-on backends that expose the detail (Apple, Linux) -- empty on Windows/Android until those
-backends populate `UrlLib`'s accessors. Browsers do not expose these properties, so
-spec-conformant code is unaffected.
+URL-opening errors populate both properties on every platform. Failures during
+`send()` expose the diagnostics supplied by the platform's `UrlLib` backend;
+those strings can be empty when the backend has no detail. Successful requests
+leave both properties empty, and reopening clears an earlier opening error.
+Browsers do not expose these properties, so spec-conformant code is unaffected.