#10246 docs(tui): align frontend protocol references

Summary

• update English and Japanese frontend README protocol claims to v12
• add a contract test that derives the version from the web client and raw protocol guide
• run the contract in the TUI spec workflow

Verification

• commit a0ec52cfd2 adds the failing contract test (red)
• commit a4a0767325 updates docs and workflow wiring (green)
• python3 cmux-tui/scripts/test_frontend_docs.py -v
• python3 -m py_compile cmux-tui/scripts/test_frontend_docs.py
• PyYAML workflow contract and actionlint
• TUI inventory, SDK schema, resource boundary, and codegen checks

No build, package publication, workflow dispatch, or merge was performed. This PR is independent of #10241 and #10242.

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Summary by cubic

Aligns frontend README protocol references to v12 and adds a contract test so docs can't drift from the supported WebSocket protocol. READMEs previously could claim an outdated protocol version with nothing enforcing it; now the spec workflow fails when they mismatch.

• Updates frontends/README.md, frontends/README.ja.md, and frontends/web/README.ja.md to reference protocol v12.
• Adds cmux-tui/scripts/test_frontend_docs.py to assert that the web client's SUPPORTED_PROTOCOL, the protocol guide heading, and README claims all match.
• Runs the new test in cmux-tui-spec.yml so version drift fails the workflow.
• On protocol bumps, update frontends/web/src/lib/protocol.ts and docs/protocol.md first, then fix README references to the new version.