_activeSessionName lacks volatile — stale reads on ARM from bridge/restore threads

[btessiau]

Problem

_activeSessionName is a plain string? field on CopilotService accessed from multiple threads:

Thread	Operation	Location
UI thread	Write (SetActiveSession, CloseSession, RenameSession, CreateSession)	CopilotService.cs
WsBridge background	Read + conditional write	CopilotService.Bridge.cs:532
Restore background	Conditional write (??=)	CopilotService.Persistence.cs:380
SDK event handler	Read comparison	CopilotService.Events.cs:876
SaveActiveSessionsToDisk	Read	CopilotService.Persistence.cs:457

On ARM (iOS/Android), without volatile, a write from the UI thread may sit in a CPU store buffer and never become visible to reads on other cores. This can cause:

• WsBridge syncing a stale active session name
• SaveActiveSessionsToDisk persisting the wrong active session
• Events handler comparing against a stale value

Proposed Fix

Mark the field volatile — ensures memory barriers on every read/write. All compound patterns (??=, compare-then-swap) are either on the UI thread or benign (idempotent/first-write-wins).