[botoddly]

Problem

XdlSharp.Native.browser-wasm added --use-port=emdawnwebgpu to the link. The port compiles webgpu.cpp into the Emscripten cache at link time, and the wasm-tools workload's cache is read-only, so every browser app had to set WasmCachePath to a writable copy of it.

Change

• XDL_VERSION is 3.4.20-9, whose browser zip ships emdawnwebgpu prebuilt: emdawnwebgpu.a and its four JavaScript libraries (Ship emdawnwebgpu prebuilt in the browser zip so .NET apps link without a writable Emscripten cache XDL#27).
• The package's targets link emdawnwebgpu.a with ScanForPInvokes="false" and pass the JavaScript libraries with --js-library in the port's order, instead of --use-port=emdawnwebgpu.
• Both browser test projects drop WasmCachePath, so CI links with the workload's read-only cache, as an app does.
• XdlBrowserDirectory moves to Directory.Build.props, because XdlSharp.Browser.Tests now reads it in a property, which Directory.Build.targets is evaluated too late for.
• The README drops the writable cache and names the two exports a page needs to hand XDL a WebGPU device.

Tested locally

• The generated bindings match the 3.4.20-9 headers (--check --clang clang-22).
• XdlSharp.Tests: 15 passed.
• XdlSharp.Browser.Tests under Node: 14 passed.
• tools/pack.sh, then XdlSharp.Package.Browser.Tests with the packed packages: all four libraries load, no archive or JavaScript library lands in the published output, and emdawnwebgpu is not in the P/Invoke table.
• The workload's Emscripten cache is unchanged after each link.