feat(native): PR1B — byte-bounded outbound queue, typed send result

[mrmidi]

PR1B — Queue and Batch Bounds

Base: pr1a/native-baseline (stacked PR chain)
fptn submodule: 13acece (pr1b/queue-bounds)

Send() — typed result + byte accounting

• Returns SendResult enum: accepted, queue_full, transport_stopped, invalid_packet
• Overflow-safe CAS byte reservation before try_send. Both bytes and packet count reserved before the channel operation.
• Rollback both counters on channel-full or exception. Channel-full increments queue_full_count_.
• iOS caps: 256 KiB queued bytes, 64 KiB batch raw bytes
• Other platforms: unlimited (existing 256-packet channel bound)

RunSender() — strict batch limit + accounting release

• releaseDequeuedPacketAccounting() called immediately on every dequeue, before IP-version validation
• Carried packet (pending_packet) prevents batch overshoot: bounds raw IP payload bytes to 64 KiB except one individually oversized packet
• Exact encoded-size control deferred to PR5

Queue counters (unsigned, never reset in Stop)

Counter	Semantics
queued_packets	Current gauge
queued_bytes	Current gauge (channel queue only; sender may additionally hold a batch + carried packet)
queued_bytes_peak	Lifetime high-water mark
queue_full_count	Lifetime byte-cap/channel-capacity rejection count

C++ → Swift boundary

• C++ sendPacket() returns uint8_t; null/zero-length rejected as invalid_packet
• Swift WebsocketSendResult enum in SharedTunnelRuntime; init(bridgeValue:) maps raw value; .unknown = 255

Provider behavior

packetLoop: for packet in packets {
    switch client?.sendPacket(packet) ?? .transportStopped {
    case .accepted:         break
    case .queueFull:        queueFullPackets += 1
    case .transportStopped: transportStopped = true; break packetLoop
    case .invalidPacket:    invalidPackets += 1
    case .unknown:          unknownResults += 1
    }
}
// totalBytes computed before send loop (full read batch)
// only queueFullPackets passed to recordPacketFlowRead()
// transportStopped → early return

Native cleanups

• #include <TargetConditionals.h> moved outside namespace
• Null/zero-length check in bridge sendPacket before vector construction

Verification

• Native framework (Debug, C++23): built
• Xcode Debug build: succeeded
• Tests: 24/24 passed

Native test debt

C++ accounting tests (concurrent CAS, rollback, gauge lifecycle, batch bounds) need fptn gtest infrastructure. Tracked for PR1B/PR1C fptn branch.

Stack

PR-1  Measurement safety
PR0   Production native build
PR1A  Native fixed baseline       (base)
PR1B  Queue and batch bounds      ← this PR
PR1C  Native teardown ownership
PR2   Wrapper/provider integration
PR3   C-based binary flight recorder