[allsey87]

Newer kernels call the boot list precompile rather than preload, and their client refuses to construct without createCanvas/destroyCanvas handlers.

• Kernel.ts boots with precompile ?? preload, so sysroots published for earlier kernels (immutable, e.g. jq) keep working.

• The canvas handlers are no-ops (createCanvas throws: test runs never draw). Older kernel clients ignore handlers they don't know, so this is safe for them too.

• test_runner_artifact forwards the S3 object's Content-Encoding: artifacts are now stored brotli-encoded (exercism/clientside-tooling#3), and these responses are no-transform, so the header is the only way the browser learns to decode them. Objects published without one (jq's) are unaffected.

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 #10692 (this PR): must be deployed before any track's boot.json uses precompile.
3. Add client-side runner cpp-test-runner#106: 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.