feat(mtmharness): add browser secondary extension ABI

[codeh007]

Summary

• Add a versioned mount(context) -> cleanup frontend extension ABI to mtmharness.
• Fetch arbitrary HTTPS artifacts, verify SHA-256, import native ESM, serialize enable/disable, and clean up owned DOM/resources.
• Convert mtmcanvas to a browser-only in-memory artifact without DSH bundle metadata, cordis.patch.yml, or Node Host output.
• Add cancellation, malformed-manifest, failed-mount, cleanup-retry, native ESM, package, and release-contract coverage.

Verification

• pnpm install --frozen-lockfile
• pnpm check
• pnpm --filter mtmharness run pack:check
• Canvas tarball packing plus packed contract and publint --strict
• npx react-doctor@latest --verbose --scope changed
• git diff --check

Release order

mtmcanvas@0.2.0 must be published before mtmharness@0.6.0; the mtmharness release gate compares local/CDN bytes and the manifest SHA-256 pin.

Refs codeh007/gomtm#664

[coderabbitai]

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