Make HasUsedToolsThisTurn volatile for ARM memory model safety

[PureWeen]

Problem

HasUsedToolsThisTurn on SessionState is read by the watchdog timer thread and SDK background threads while being written from the UI thread and SDK event handlers. It had inconsistent synchronization: 5 sites used Volatile.Read/Write while 16 used plain access.

On ARM (iOS/Android), plain writes from the UI thread may not be immediately visible to background thread reads. This is a theoretical data race that could cause the watchdog to use incorrect timeouts (120s vs 600s).

Identified during code review of PR #342 (INV-7 from the processing-state-safety skill).

Fix

• Declare the field volatile bool — all reads/writes automatically have correct memory barriers
• Remove the 5 now-redundant Volatile.Read/Write calls (would generate CS0420 warnings)
• Remove a duplicate write in the reconnect path (line ~2613 was redundant with ~2639)
• Replace the HasUsedToolsThisTurn_VolatileConsistency test with HasUsedToolsThisTurn_IsDeclaredVolatile that uses reflection to verify the volatile modifier

Testing

All 2401 tests pass. No CS0420 warnings.

[PureWeen]

PR #344 Review — Make HasUsedToolsThisTurn volatile for ARM memory model safety

CI Status: No CI checks on this branch (all 2401 existing tests pass per PR description)
Existing reviews: None
Models: claude-opus-4.6 ×2, claude-sonnet-4.6, gemini-3-pro-preview, gpt-5.3-codex (5/5 — 3 completed, 2 in-progress at synthesis time; all 3 completed: CLEAN)

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✅ No consensus bugs found

All three completed models agree the PR is correct:

• volatile bool vs Volatile.Read/Write — semantically identical (both emit acquire-fence on read, release-fence on write). The substitution is correct at all 5 sites. ✅
• Deleted newState.HasUsedToolsThisTurn = false — newState is a freshly allocated new SessionState { Session=..., Info=... }. The bool defaults to false, so the explicit reset was dead code. The actual behavioral reset at ~line 2662 (state.HasUsedToolsThisTurn = false before StartProcessingWatchdog) remains and is the real one. ✅
• Reflection test — GetRequiredCustomModifiers().Any(m => m == typeof(IsVolatile)) correctly verifies the C# volatile modifier (emitted as modreq(IsVolatile) in IL). ✅
• CS0420 warnings — all ref state.HasUsedToolsThisTurn uses removed, so no warnings remain. ✅

One harmless nit (no action needed): the first assertion in the test — field.FieldType == typeof(bool) is always true, making the || (FieldAttributes.NotSerialized) arm unreachable. The important second assertion (IsVolatile check) is what matters and is correct.

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✅ Approve

Clean mechanical fix. The volatile declaration is the right approach for this pattern (directional bool: SDK threads write true, UI thread writes false, watchdog reads). No bugs found across models.