fix: make _activeSessionName volatile for cross-thread visibility

[btessiau]

Problem

_activeSessionName is a plain string? accessed from multiple threads without memory barriers:

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

On ARM (iOS/Android), without volatile, writes may sit in a CPU store buffer and never become visible to reads on other cores — causing stale active session name in bridge sync, persistence, and event handling.

Fix

- private string? _activeSessionName;
+ private volatile string? _activeSessionName;

Applied to both CopilotService and DemoService.

All compound patterns (??=, compare-then-swap in Close/Rename) are either on the UI thread or benign (first-write-wins on restore/bridge sync).

Testing

• 2 new reflection-based guard tests in VolatileFieldGuardTests.cs verify the volatile modifier is present (same pattern as PR Make HasUsedToolsThisTurn volatile for ARM memory model safety #344 for HasUsedToolsThisTurn)
• All existing tests pass

Closes #348

[btessiau]

🔍 PR #349 Review -- _activeSessionName volatile for cross-thread visibility

CI Status: ⚠️ No checks configured
Prior reviews: None
Diff: 3 files, +39 -2 (minimal surgical change)

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Review Summary

The change is correct, minimal, and well-justified. No blocking issues found.

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Correctness Analysis

volatile string? is valid and effective here. In C#, volatile on a reference type provides acquire/release memory barrier semantics — reads are acquire reads, writes are release writes. This prevents CPU store-buffer reordering on ARM (iOS/Android) where writes from the UI thread may not be visible to background threads without a barrier.

Compound patterns are safe:

• ??= (CopilotService.cs:1536, Persistence.cs:374, DemoService.cs:37) — These are first-write-wins races. If both UI thread and restore/bridge thread race to set the active session name from null, the last writer wins, but both values are valid session names (not corrupted data). volatile reduces the race window significantly by ensuring readers see fresh writes. This is the correct analysis in the PR description.

• if (_activeSessionName == null && ...) _activeSessionName = remoteActive; (Bridge.cs:532) — Runs on the WsBridge background thread, once on first sync. With volatile, the null check correctly sees any UI-thread write, making the race window negligible. If it fires anyway, first-write-wins is acceptable.

• if (_activeSessionName == oldName) _activeSessionName = newName; (CopilotService.cs:2975, 3015, 3118) — These run on the UI thread exclusively (CloseSession, RenameSession). The background thread never mutates _activeSessionName in the rename/close paths. volatile ensures background readers see the rename/close immediately.

• Events.cs:797 if (state.Info.Name == _activeSessionName) — Read-only use for marking messages as read. A stale read here simply means "read" badge lags one cycle. Benign.

No deadlock risk. volatile uses no locks.

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Test Quality

VolatileFieldGuardTests uses field.GetRequiredCustomModifiers().Any(m => m == typeof(IsVolatile)) — this is the correct and idiomatic way to verify volatile via reflection in .NET. The tests serve as a reliable regression guard against accidental removal of the modifier. Both CopilotService and DemoService are compiled into the test project via <Compile Include> in the csproj, so typeof(CopilotService) resolves correctly.

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Minor Observation (not blocking)

IsRestoring is a property { get; private set; } read by the SaveActiveSessionsToDisk timer callback (Persistence.cs:15, 54) on a background thread without a memory barrier. This is a pre-existing issue outside this PR's scope — the consequence is benign (at worst the timer fires once during restore and writes a partial snapshot, which is overwritten when restore completes). Worth a follow-up but not a blocker here.

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Verdict: ✅ Approve

Three-line change, correct analysis, reflection-based regression tests. Ready to merge.