Bounded multi-session worker pool

[AetherAI3]

Goal

Design and implement a bounded multi-session worker pool while preserving the current authority,
egress, resource, and human-observation boundaries.

Required design work

• Define explicit maximum capacity and admission/back-pressure behavior.
• Specify per-session ownership, controller/observer authorization, and cleanup semantics.
• Choose the browser process/context isolation model and document its security tradeoffs.
• Bound CPU, memory, storage, snapshots, and vision budgets per session and for the whole pool.
• Ensure one session cannot observe or control another session's state or noVNC surface.
• Define crash recovery and stale-session reclamation without silently reusing authority.

Acceptance criteria

• An approved design document precedes runtime implementation.
• Concurrency, exhaustion, isolation, crash, cleanup, and cross-session authorization tests exist.
• Default capacity remains conservative and fail-closed.
• Existing single-session API behavior remains compatible or receives an explicit versioned change.
• Exact-image staging acceptance demonstrates bounded resource behavior before release.

This issue must not be implemented by simply removing the current single-session guard.