test(web): preserve safe custom-element attribute defaults

[NWYLZW]

Summary

• Add a regression test for the public @oneworks/avatar-web custom-element attribute boundary.
• Verify unsupported runtime themes fall back to system, unsupported editor locales fall back to en, boolean attributes preserve HTML presence semantics, and valid dark theme propagates.

Why this is safe

This is a source-only safety-boundary regression in the Web Components adapter. The change is one regular packages/ test file; no CI trust-boundary, evidence verifier, deploy, npm, version, lockfile, generated asset, or documentation file is changed.

Hosted reuse canary

The PR must receive the normal full Avatar SDK CI validation and produce its exact integration-tree evidence artifact. If this exact integration tree is squash-merged without base/tree/first-parent drift, protected main may reuse that artifact as reused-pr-ci. Direct/manual events, trust-file changes, tampered evidence, and identity/tree/parent mismatches must remain full or fail-closed.

No hosted workflow or production reuse result has been observed yet; this PR body does not claim one.

Validation

• pnpm exec vitest run packages/web/__tests__/elements.spec.ts packages/web/__tests__/lifecycle.spec.ts packages/web/__tests__/pickers.spec.ts
• pnpm exec vitest run __tests__/prCiEvidence.spec.ts
• pnpm typecheck
• Node/YAML syntax and git diff --check

Review scope

• Base locked to live protected main at d0b7936873f0e855b1724577b771196450218c9a before branch creation.
• One-file regular A/M diff; no CI/release-path edits.
• Follow-up acceptance requires PR full validation, exact artifact identity, squash merge with unchanged base/tree/first parent, main reused-pr-ci, and Deploy Avatar exact-SHA success.