fix: docs build failure and add local example validation

[cfoust]

Summary

Fixes the docs build CI failure and adds a local command to run CI's Janet example validation.

• pkg/cy/api/docs-signal.md — the signal/wait example blocks forever (nothing sends :my-channel in a standalone run), which timed out the example validator and failed the build. Marked # ignore like other blocking examples. The non-blocking signal/send example stays validated.
• docs/docs/layouts.md — fixed a dynamic-property arity mismatch. The :title property now passes (size, layout), so border-title takes [size layout]. border-fg is preset with only the layout, so it stays [layout]. This clears the function takes at most 1 args, got 2 log lines.
• justfile — added just docs-validate, which runs CI's exact example check locally. It clears both docs/.docusaurus and docs/node_modules/.cache so the markdown preprocessor actually re-runs; a stale webpack cache otherwise reuses processed output and silently skips validation.

Test plan

• just docs-validate passes: 72 examples validated, 0 property errors.
• Removing # ignore reproduces the exact CI failure (api.md:… timed out waiting for signal "my-channel", exit 1); restoring it passes.

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