P1.1: Add MultiplexerBackend interface (multiplexer abstraction Phase 1)

[takemo101]

Phase 1, issue P1.1 — part of the multiplexer backend abstraction implementation plan.

Design: docs/designs/cuekit-multiplexer-backend-design.md

Prerequisites: none. Can start any time.

Outcome

@cuekit/adapters exports MultiplexerBackend, SpawnPaneParams, PaneHandle, CaptureOptions types. No implementation, no callers changed.

Files

• Create: packages/adapters/src/multiplexer-backend.ts
• Modify: packages/adapters/src/index.ts (re-export)

Steps

• Define interface and supporting types per the design doc's "Proposed MultiplexerBackend interface" section.
• Add JSDoc on each method describing the contract (especially sendKeys newline simulation and capturePane null-on-failure).
• Re-export from index.ts.
• No tests in this issue; types alone.

Acceptance

bun test --pass-with-no-tests and bun run typecheck succeed; no callers reference the new interface yet.