[browser][coreCLR] Fix download() → create() lifecycle for WASM loader

[pavelsavara]

Fixes #124896

The download() API was broken — calling download() followed by create() failed because the native module couldn't be re-initialized. This PR restructures the createRuntime() flow to properly support the two-phase download→create lifecycle.

Changes

Loader core (run.ts)

• Track download/config state with module-level flags (downloadStarted, downloadedIntoMemory, configInitialized) so download() and create() can be called sequentially without re-running initialization.
• Config initialization (onConfigLoaded, modulesAfterConfigLoaded, polyfills, console wiring) now runs exactly once regardless of how many times createRuntime is entered.
• When download() already loaded assets into memory, create() takes a fast path: initialize CoreCLR, fire onDotnetReady, load modulesAfterRuntimeReady — skipping redundant downloads.
• Duplicate download() calls are a no-op.

HTTP-cache-only prefetch mode (assets.ts)

• New download(httpCacheOnly: true) parameter. When set, resources are fetched into the browser HTTP cache and discarded — no WASM memory is consumed. A subsequent create() will re-fetch from cache.
• prefetchAllResources() enqueues all data assets (assemblies, ICU, VFS, WASM, PDBs, symbols, satellites) and fetches them with throttling (maxParallelDownloads).
• prefetchJSModuleLinks() injects <link rel="prefetch"> hints for JS modules instead of fetching them, since they use import().
• prefetchUrl() respects loadBootResourceCallback, integrity checks, and disableNoCacheFetch.

Public API (public-api.ts, dotnet.d.ts, host-builder.ts)

• download() signature updated to download(httpCacheOnly?: boolean).

Tests

• Extended NoResourcesReFetchedAfterDownloadFinished to verify onConfigLoaded fires during download(), resources are fetched during download, none re-fetched during create(), and create() completes.
• New HttpCacheOnlyThenCreateWorks test covering download(true) → create() with loadBootResource callback verification.
• Added test scenarios DownloadThenInitHttpCacheOnly in main.js.
• Enabled DownloadThenInitTests for CoreCLR in BuildWasmAppsJobsListCoreCLR.txt.

[Copilot]

Pull request overview

Restructures the WASM loader lifecycle to support a two-phase download() → create() flow (including an HTTP-cache-only prefetch mode), and extends coverage to prevent regressions.

Changes:

• Adds loader state tracking so download() can be followed by create() without re-running one-time initialization.
• Introduces download(httpCacheOnly?: boolean) and implements HTTP-cache-only prefetching for boot assets (plus <link rel="prefetch"> hints for JS modules).
• Extends/introduces WASM build tests and enables the previously-disabled CoreCLR scenario.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/native/libs/Common/JavaScript/types/public-api.ts	Updates the public download(httpCacheOnly?: boolean) API signature and docs.
src/native/libs/Common/JavaScript/loader/run.ts	Implements the new lifecycle/state machine and cache-only download path.
src/native/libs/Common/JavaScript/loader/host-builder.ts	Wires download(httpCacheOnly?: boolean) through to createRuntime.
src/native/libs/Common/JavaScript/loader/dotnet.d.ts	Updates TypeScript declarations for the new download signature.
src/native/libs/Common/JavaScript/loader/assets.ts	Adds HTTP-cache-only prefetch implementation and JS module prefetch link hints.
src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js	Adds browser test scenarios for download()→create() and download(true)→create().
src/mono/wasm/Wasm.Build.Tests/DownloadThenInitTests.cs	Extends existing test and adds a new test validating the cache-only mode.
eng/testing/scenarios/BuildWasmAppsJobsListCoreCLR.txt	Enables DownloadThenInitTests for CoreCLR scenario runs.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 4 comments.

[maraf]

Looks good to me and my copilot 👍