feat: PR3 — native binary diagnostics core (C ABI + C++23)

[mrmidi]

PR3 — Native Binary Diagnostics Core

Base: pr2/provider-integration · Head: 76ac6e4 · 17 files, +1739/-98

C++ core (TunnelDiagnosticsCore static library)

• C++23, no exceptions, no RTTI, C-compatible ABI (void* handles)
• RAII: PosixFile, FlightRecorder, LifecycleStore
• Explicit little-endian byte serialization + CRC32 (no struct dumps)
• 72-byte flight events, 120-record ring (8,704 bytes fixed)
• Double-buffered lifecycle snapshot (2 × 512 bytes)
• Thread-safe recorder (internal mutex), reopen recovery from max sequence

Provider integration — complete

• All 38+ call sites use explicit flightEvent: codes (mandatory parameter, no fallback mapping)
• recordProviderEvent → flightRecorder.record() (numeric, @autoclosure message never evaluated)
• updateDiagnosticsHeartbeat → writeLifecycleSnapshot() (binary snapshot, no JSON)
• Zero JSONL writes, zero TunnelDiagnosticsStore calls from the provider
• diagnosticsLock protects latestEventSequence and physicalFootprintPeakBytes
• wsClient and lastNativeStatus copied under stateLock, status queried outside lock
• LocalStopInitiator.binaryCode (1/2/3) and TunnelRuntimeState.binaryCode (0-6) — no hashValue
• All snapshot fields populated: tunnelStartedMachTime, lifecycleState, providerStopReason, footprintPeak, activeBridges, activeReadLoops, latestEventSequence, packetReadPending flag, lastNativeStatus fallback, Mach memory fallback

What was removed

• JSONL event writes from provider
• JSON heartbeat writes from provider
• flightEventCode(for:) category mapping (explicit codes are the contract)
• updateDiagnosticsHeartbeat(snapshot:lastEvent:) overload (no double currentSnapshot())
• #if FPTN_MEASUREMENT_BUILD gates on diagnostics (no JSONL to gate)

Kept (app-side, unrelated)

• MetricKit records
• Crash signal installer (best-effort)
• App-side JSON readers (for old files on disk during transition)

Verification

• C++ core: builds for iOS (arm64)
• Xcode Debug build: succeeded
• Tests: 27/27 passed
• Mandatory flightEvent:: compiler enforces — no call site can regress to unmapped events

Remaining (follow-up PRs)

• App-side decoder (TunnelDiagnosticsDecoder)
• VPNService fallback → binary snapshot
• LogsViewModel export → binary decoder
• Remove obsolete JSON provider readers/models

Stack

PR-1  → master                    #1
PR0   → pr-1/measurement-safety   #2
PR1A  → pr0/native-build          #3
PR1B  → pr1a/native-baseline      #4
PR1C  → pr1b/queue-bounds         #5
PR2   → pr1c/teardown-ownership   #6
PR3   → pr2/provider-integration  #7  ← this PR
PR3A  → pr3                       next: Instruments signposts