Add code coverage and ioctl unit testing

[colemancda]

Summary

Adds code coverage measurement/enforcement and unit-testing scaffolding for the package's ioctl() calls, plus value-type tests — and fixes three real bugs the new tests exposed.

Coverage: 1.53% → 41.23% (949/2302 lines), 40 tests, all passing. Rebased on latest master.

Code coverage

• Scripts/coverage.sh runs the suite with coverage, exports LCOV + Cobertura reports scoped to the package's own Sources/, and enforces a minimum line-coverage threshold (currently 1%, to be ratcheted up as the suite grows).
• CI (swift.yml) runs the gate and publishes to GitHub Code Quality; replaces the retired maxep/spm-lcov-action step and refreshes the deprecated ubuntu-20.04 runner and upload-artifact@v3 action.

ioctl unit testing

Syscall mocking modeled on swift-system's MockingDriver:

• Internal/Mocking.swift — thread-local MockingDriver that records a trace of syscalls, can force errnos (.always / .counted for error-path and EINTR-retry tests), and exposes an ioctlHandler closure to fake the kernel's out-parameter reply.
• Internal/Syscalls.swift — system_ioctl shims that divert to the mock under ENABLE_MOCKING (debug only) and compile to the plain ioctl in release.
• Internal/IOControl.swift — in-module SocketDescriptor.inputOutput overloads that route all existing ioctl call sites through the mockable seam without touching them.
• ENABLE_MOCKING is defined for debug builds only, mirroring swift-system.

Bugs found and fixed by the new tests

• HCIScan inquiry parsing used the wrong buffer offset — results were read at Swift MemoryLayout.size (9) instead of the C sizeof(struct hci_inquiry_req) (10) the kernel writes at, and the misaligned typed rebind traps at runtime. Rewritten with a properly aligned raw buffer, stride-based offsets, and loadUnaligned.
• RFCOMMIO.GetDeviceList had the same ABI bug — dev_info[] was read at offset 2 instead of the C flexible-array offset 4 (count padded to element alignment), with the same misalignment trap.
• RFCOMMDevice copied the source address into destination (destination = cValue.source).

Tests

• IOControlTests.swift — ioctl paths through the mock seam: request tracing, faked kernel replies for HCIGETDEVLIST, HCIGETDEVINFO, HCIINQUIRY (verifies marshalled request fields incl. GIAC lap), RFCOMM create/release/list/get-info, forced-errno, and interrupt-retry paths.
• ValueTests.swift — pure value-type logic: AddressType, the full HCIError → Errno mapping, HostControllerIO/RFCOMMIO round trips + descriptions + Codable, HCIDeviceOptions, LinkMode, RFCOMMState, HCIBusType/HCIControllerType descriptions, HCIPacketType, BluetoothSocketProtocol, and socket-option encode/decode round trips (Security, RFCOMM LinkMode, ConnectionInfo).

All run without Bluetooth hardware, on Linux and Darwin.

Progress log

Milestone	Coverage	Tests
Initial ioctl mocking scaffolding	12.45%	9
Value-type + ioctl device/RFCOMM tests	26.42%	26
Inquiry/RFCOMM-list ioctls, socket options, ABI fixes	41.23%	40

Now fully covered: Error, AddressType, HCIBusType, HCIControllerType, HCIDeviceFlag, HCIDeviceOptions, RFCOMMFlag, RFCOMMLinkMode, RFCOMMGetDeviceInformation. Remaining gaps are the async HostController operations and L2CAP/socket plumbing that need a live HCI socket.

The toolchain is pinned to Swift 6.3.3 via .swift-version.

Includes the BitMaskOptionSet → native OptionSet migration (#53, merged into this branch).