Add a native plugin right-panel extension API

[brsbl]

Decision

Use one structured, experimental right-panel API because the real plugin cases share host-owned chrome and lifecycle: custom navigation views, Browser tabs, and Terminal tabs beside a plugin nav page. Keep thread panels separate because their ownership and lifecycle are thread-scoped rather than nav-page-scoped.

API

• navPanel.experimental_rightPanel registers plugin views, an optional pinned defaultViewId, and enabled host tools (browser, terminal)
• custom views receive { subPath, params } and choose padded or flush layout
• useBbNavigate().experimental_openRightPanel(...) opens a registered view, Browser URL, or Terminal target and returns false for invalid or unavailable requests
• BB owns resize behavior, compact drawers, tabs, chrome, persistence, focus, and terminal cleanup

Implementation

• host custom views and native Browser or Terminal tabs in one BB right-panel deck
• preserve pane-scoped focus and maximize visibility, final-tab teardown, restored state, and compact/standalone surfaces
• validate registration and runtime requests before access or mutation
• cover view/tool registration, lifecycle, malformed JavaScript input, terminal retry/cleanup, navigation, and native Browser ordering
• update SDK declarations, testing harness, starter declarations, authoring guidance, and API audit

Verification

• focused app regressions: 118 tests
• plugin SDK: 77 tests
• templates: 23 tests
• app, plugin SDK, and template typechecks
• real standalone BB desktop verification of plugin/right-panel alignment and Browser side-by-side behavior

This upstream-origin PR supersedes fork-origin #1497 without folding any plugin implementation into BB.

BB-Thread-ID: thr_ehwczw5k36