Fix canvas owner liveness across concurrent sessions

[0101]

Problem

• Two Copilot sessions can remain active in the same worktree and heartbeat independently.
• The bridge liveness API exposed only the most recently registered session ID for each worktree.
• AgentDoc liveness dots and Start-session visibility compared ownership against that alternating ID, so a healthy document owner appeared offline whenever the other session sent the latest heartbeat.

Fix

• Add BridgeLiveness.LiveSessionIds containing every identified session still within the liveness TTL.
• Check AgentDoc ownership against the complete live-session set.
• Preserve the existing freshest SessionId for aggregate status and SystemView fallback routing.
• Update the canvas-pane specification and add regression coverage for a non-freshest live owner.

Tests

• dotnet test src\Tests\Tests.fsproj --filter "FullyQualifiedName~CanvasBridgeTests|FullyQualifiedName~SessionBridgeTests" — 66 passed
• npm run build

[Copilot]

Pull request overview

Extends bridge liveness tracking so concurrently active sessions keep their AgentDocs online independently.

Changes:

• Adds and populates all live session IDs per worktree.
• Uses the full live-session set for document status and session-launch visibility.
• Updates specifications and regression tests.

Show a summary per file

File	Description
src/Shared/Types.fs	Adds LiveSessionIds and lookup helper.
src/Server/SessionBridge.fs	Aggregates live identified sessions.
src/Client/CanvasPane.fs	Checks document owners against all live sessions.
src/Tests/SessionBridgeTests.fs	Covers stale-session liveness output.
src/Tests/CanvasBridgeTests.fs	Covers concurrent non-freshest owners.
docs/spec/canvas-pane.md	Documents multi-session liveness behavior.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Balanced