Client: keep more than one session live at a time (multi-pane conversation rendering)

[icesword0760]

What I ran into

I'm building an external layout plugin that renders several conversations side by
side — each pane a complete session with its own transcript, composer and approval
flow.

The client stage is a single watched id that follows list.current
(packages/api/session-controller/src/client/sessions/service.ts), so exactly one
session's event window is open at a time. Non-current panes fall back to the current
binding and show the wrong transcript. That file's own module comment already
reserves this evolution — "the staged state can widen to a multi-pane list later" —
which is what led me to try it.

What I did locally

Three small changes, kept deliberately additive:

1. session-controller (client) — widen the stage from a single watched id to
   current ∪ pinned. New ISessions.pin/unpin/staged and SessionListState.staged;
   pruneScopes/sweepDeferred decide by set membership. open() still moves the
   single selection, so the selection stays single — staged is a projection for
   renderers, not a second selection.
2. ui-slots / ui-session — an optional SlotScopeAdapter.observe?(key): an
   observable over one session's materialized binding, so a subtree rendered against a
   pinned, non-current session never holds a stale binding.
3. ui-slots / ui-renderer — RenderOpts.sessionKey /
   SessionAreaProps.sessionKey, binding a renderSlot subtree to one session
   identity instead of the current selection. Prefers observe when installed, falls
   back to resolve(); an unknown key falls back to the current binding.

Single-stage behaviour is unchanged. With nothing pinned, current ∪ pinned
reduces to current and every existing path behaves exactly as before. Both rendering
additions are optional and fall back to today's behaviour when absent.

Verification

Rebased onto 0.1.5-rc.2. tsc -b tsconfig.client.json is clean and the full
workspace suite is green: 412 test files, 6110 tests.

38 files change. Eight are source — seven product files plus one test-support double —
and the remaining thirty are test fixtures picking up the three new interface members.

Four new unit tests cover the keyed scope, keyed rendering and keyed observation. To be
straight about the gap: the staged set itself (pin/unpin) is currently exercised
only through the existing stage-lifecycle tests, not by dedicated ones.

Why I'm posting instead of opening a PR

CONTRIBUTING says external PRs aren't being accepted right now — understood, no
problem. I'm posting in case the capability is interesting to the team, and so other
plugin authors hitting the same wall can find this thread. Happy to share the patch
series if that's useful.

The ask

Some way for a plugin to keep more than one session's event window open. The shape
above is just what worked for me; anything that lets a renderer bind a subtree to a
session other than the current selection would solve it.