Add plugin timeline annotation host contracts

[brsbl]

Summary

Add the narrow BB host and plugin SDK contracts required by selection-bound timeline annotations:

• let message actions opt into the floating selection menu without appearing in the per-message action bar
• capture rendered-text UTF-16 selectors, quote context, and line-fragment geometry from user and assistant prose
• expose experimental native timeline DOM hooks without enabling annotation inside embedded plugin chats
• give lifecycle-managed content scripts experimental plugin-scoped RPC, realtime, connection-state, and composer navigation capabilities
• let thread-panel plugins experimentally reveal a native message, including bounded older-history loading, before resolving its exact text range
• update the SDK harness, generated declarations, templates, plugin-authoring documentation, and API audit ledger

Every new public member uses an experimental_ prefix. Native DOM hooks use the equivalent data-bb-experimental-* namespace.

Why

This unblocks the separate Timeline Comments plugin PR without moving plugin-owned annotation data or UI into BB core. BB supplies only the selection, lifecycle transport, DOM, and reveal primitives that an external plugin cannot infer safely.

Validation

• 105 focused app tests
• 75 plugin SDK tests
• 14 plugin-authoring documentation tests
• 13 template and external-scaffold tests
• scoped app/server/SDK/templates typecheck passed
• app lint passed with zero errors and the repository's existing warnings only
• generated SDK declarations and templates are current
• git diff --check