Add C ABI

[colemancda]

Summary

Scaffolding for a drop-in, Swift-implemented replacement for BlueZ's shared libbluetooth.so.3. The deliverable is a library that loads, resolves all 218 symbols the reference exports, and fails loudly (by symbol name) on any call not implemented yet — so the rest of the port can proceed incrementally, one symbol family at a time.

Nothing in the bluez package actually links this shared library — bluetoothd, bluetoothctl, btmon, hciconfig and sdptool all statically link libbluetooth-internal.a — so replacing it can't break the Bluetooth stack itself; it exists solely for third-party consumers.

• Sources/CBluetoothLinuxABI vendors the eleven public BlueZ headers verbatim (GPL-2.0-or-later, isolated in its own directory with its own LICENSE and README) and generates a stub for every symbol not yet implemented, via scripts/gen_stubs.py. Each stub's signature is read out of the vendored headers, so it's checked against the real declaration rather than hand-typed.
• scripts/symbols.txt records the reference export surface (218, split 17/101/100 across bluetooth.c/hci.c/sdp.c); scripts/implemented.txt records the 27 currently implemented (the bluetooth.c + bt_uuid_* families, implemented in PureSwift/Bluetooth). scripts/gen_symbols.py derives the linker version script and the flat export list from these; scripts/check-exports.sh asserts the built library against it.
• scripts/ownership.md records the return/who-frees/lifetime contract for each symbol, ahead of implementation — the conventions aren't uniform (some allocate on the heap, some write into a caller buffer, some return static pointers) and guessing produces leaks or double frees no type checker would catch.
• CMakeLists.txt builds libbluetooth.so.3.19.15 by linking a PureSwift/Bluetooth checkout's static archives (-DBLUETOOTH_PACKAGE_PATH) together with the generated stubs, with the soname, version, and a pinned export list that Package.swift can't express.
• Conformance/README.md records where the differential conformance harness for the implemented symbols lives (in PureSwift/Bluetooth, alongside the symbols it covers) and what's needed for the rest.

Verified

• The built library exports exactly the 228 expected symbols (check-exports target).
• Its output over the differential conformance driver for the implemented symbols is identical to the system libbluetooth.so.3, aside from three already-documented bt_compidtostr naming deltas.
• Calling an unimplemented symbol (e.g. hci_open_dev) aborts with libbluetooth (PureSwift): hci_open_dev is not implemented yet. rather than crashing unhelpfully or corrupting state.

Test plan

• swift build — default configuration, unaffected
• SWIFTPM_BLUETOOTH_CABI=1 swift build — C ABI target builds
• cmake -B build -G Ninja -DBLUETOOTH_PACKAGE_PATH=<Bluetooth checkout> && cmake --build build — libbluetooth.so.3.19.15 builds
• cmake --build build --target check-exports — 228/228 symbols match
• Smoke test: reference and built library produce identical output over the implemented symbols' conformance driver
• Smoke test: an unimplemented symbol aborts loudly by name instead of misbehaving