Add client-side runner

[allsey87]

Runs bin/run.sh in the browser on the wasm kernel, as jq-test-runner does.

• exercism_parser is compiled in GHA inside the kernel by the sysroot's clang, linked without --export-all and run through wasm-opt (55KB, reproducible). bin/build-clientside-parser.sh does this.

• Publish runs the suite in the kernel against the exact tarball it uploads.

• run-in-kernel.mjs gains --extract, supports precompile and the canvas handlers newer kernels require.

• bin/run-tests.sh now fails when results differ ($? was read after rm), and compares build errors by status only, since GCC and clang word them differently.

• helpers.h drops std::ptr_fun (removed in C++17, absent from libc++). example-tbb and libtbb-dev are removed.

• The workflows fetch with curl --compressed (artifacts are stored brotli-encoded), and run-in-kernel.mjs passes the kernel's fetches of stubbed binaries (/test-runners/...overlay/...) through to exercism.org, so CI runs on exactly what students get.

The client-side check needs exercism/clientside-tooling#3 merged and published first.

Closes #104

Merge order

1. https://github.com/exercism/clientside-tooling/pull/3: publishes the cpp kernel and sysroot.
2. Accept precompile in boot.json, add canvas handlers website#10692: must be deployed before any track's boot.json uses precompile.
3. Add client-side runner #106 (this PR): its client-side check needs 1 published and 2 deployed (it fetches the brotli-encoded objects), and its first publish writes cpp/latest.json, so it merges last.

[vaeng]

Am I right to assume that you still work ok that, until all tests are green?

[allsey87]

I think the test is only red because of the dependent PRs