Add nav panel menu example plugin

[brsbl]

What was wrong

The new optional plugin-page menu contract had no in-repository consumer, so its group ordering, context helpers, and lazy-submenu shape could regress without exercising the same API a real plugin would import.

What changed

• Adds examples/plugins/nav-panel-menu, a self-contained plugin that registers two labeled groups and a lazy API surfaces submenu through experimental_menu.
• Exercises MenuContext.navigate and openInSplit from real menu actions while keeping lazy submenu items unresolved until the submenu opens.
• Gives only this new consumer the capability-based engines.bbPluginSdk >=0.4.18 floor; unrelated examples and the shared released baseline are unchanged.
• Adds focused rendering and lazy-resolution tests, a frozen lockfile importer, and the example-specific Turbo typecheck edge to the layer-1 SDK declarations. No other bb behavior changes in this layer.

Before — parent layer dd32116ce

After — this PR head 0c8d9af82

How you verified

• Frozen offline lockfile validation passed with pnpm install --lockfile-only --frozen-lockfile --ignore-scripts --offline against an isolated modules directory.
• pnpm exec oxfmt --threads=1 --check examples/plugins/nav-panel-menu passed.
• pnpm exec oxlint examples/plugins/nav-panel-menu passed.
• Focused Vitest passed: 1 file, 2 tests.
• Focused source-contract TypeScript validation passed against the layer-1 @get-bb/plugin-sdk/app source in 9.7 seconds.
• The real plugin installed as running against the exact SDK 0.4.18 branch app; its two groups rendered, the lazy submenu produced Agents / App slots / Storage only on open, and selecting Agents navigated to surfaces/agents without a console exception.
• The full Turbo typecheck correctly reached the SDK declaration build but remained CPU-bound on this 8 GB host at one worker and was stopped after more than two minutes; CI must run the complete check.
• Chrome for Testing 152.0.7977.54 passed the cumulative responsive, theme, persistence, active-promotion, right-click, and repeated-disclosure flow. Safari 18.2 remains unverified because Allow remote automation is disabled on this host.

Fixes

No linked issue; supplies the required living consumer layer for Phase 3 of the Updated Sidebar Spec.

BB-Thread-ID: thr_jgmy8ay2qd

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