wasm32-emscripten: per-OS descriptor sections fall back to the host's, release links without -O, only JS exceptions #851

Found while building openmui's dependencies for wasm32-emscripten (mcpp 2026.10.26.1, emsdk 6.0.9, node 26.7.0). Three engine gaps, in order of impact.

1. An Emscripten build reads the host's per-OS section of a package descriptor

TargetPlatform::for_os (modules/toolchain-model/src/platform.cppm) maps a triple's os through xpkg_platform_key_for, which knows linux, macos(x) and windows; for emscripten it returns nullopt and for_os falls back to the host (value_or(xpkg_platform)).

So on a Linux host a wasm build takes a descriptor's linux block:

• taskflow.taskflow's Linux -pthread is applied. Without [target.'cfg(os = "emscripten")'.abi] threads = true the link fails with --shared-memory is disallowed, although the program never asked for threads.
• compat.freetype's Linux sources and flags are compiled into the wasm build.

On a Windows host the same build would take the windows block (e.g. FreeType's builds/windows/ftsystem.c) — inferred from the descriptor and the code above, not run.

Reproduce: a package depending on taskflow.taskflow = "4.1.0" that uses nothing but tf::Taskflow, then mcpp build --target wasm32-emscripten on Linux.

What would fix it: a descriptor key of its own for Emscripten (absent block = the package declares no Emscripten build) instead of the host fallback, so a target never silently inherits the build machine's platform.

2. A release build links without an optimisation level

The link line carries no -O, so emcc skips wasm-opt and leaves the JavaScript launcher unminified. With ldflags = ["-O2"] added for Emscripten:

The profile's optimisation level should reach the link for this target (for emcc the link is where wasm-opt runs).

3. Exceptions: only the JavaScript-based model

[target.'cfg(os = "emscripten")'.abi] exceptions = true selects -fexceptions (JS-based). Native wasm exceptions (-fwasm-exceptions) are smaller and faster and are what a new build should prefer; there is no way to choose them.

Also observed (defaults, perhaps documentation)

Emscripten's defaults are a 64 KiB stack and a fixed 16 MiB heap: a 64 MiB allocation aborts with "OOM", and a 64 KiB buffer on the stack overflows it. ldflags = ["-sALLOW_MEMORY_GROWTH=1", "-sSTACK_SIZE=1048576"] fixes both; worth either a documented recipe or [target.<sel>.abi] keys.