feat(artifacts): add production AgentReact runtime

[phodal]

Summary

• add a spec-backed AgentReact pipeline from immutable source revisions through Oxc semantic compilation, confined linking, and build snapshots
• register explicit *.agent.canvas.tsx artifacts with an opaque iframe Current/Staging transaction and Host-owned state, Actions, observations, and node addressing
• run production Oxc compilation in a restartable Worker with deadlines and complete the optional native-binding lockfile closure
• preserve the committed AgentReact session across unrelated artifact invalidations and support static barrel re-exports

Traceability

• Spec: docs/specs/2026-08-27-agent-react-artifact-runtime-poc.md
• Acceptance: AR-AC-1 through AR-AC-30
• No external Story or Issue was provided; none is inferred.

Validation

• npm run check: root 1545 passed with 2 skipped; Harness 173; Harness UI 31; Studio 466; generated-source and pack verification passed
• npm run test:browser: 47 Chromium tests passed, including wide, compact, narrow, staging rollback, runtime failure recovery, and repeated unrelated invalidations
• focused AgentReact and production compile suite: 207 tests across 14 files passed
• Intel macOS install plan selects both @oxc-parser/binding-darwin-x64@0.147.0 and @oxc-transform/binding-darwin-x64@0.147.0
• staged diff whitespace check passed before commit

Risk and delivery boundaries

• Oxc adds platform-specific native optional dependencies; versions and integrity are pinned and the compiler remains behind OxcCompilerPort.
• Local runtime and extracted-install evidence is macOS arm64. Intel macOS evidence covers dependency closure only; Windows and Linux runtime claims require their corresponding CI jobs.
• Publication, release, and tag creation are not part of this PR.