feat(pkg): add LibSerial 1.0.0 compat package

[tz12323]

Add compat.libserial, a Linux-only C++-source compat package for LibSerial 1.0.0 (SerialPort / SerialStream), built from upstream's LIBSERIAL_SOURCES three translation units and exposing the public headers under libserial/ the way upstream's install does.

The headers open with <termios.h> and every TU reaches <linux/serial.h>, so the descriptor declares only the linux platform and consumers gate it with [target.'cfg(linux)'.dependencies] (same shape as compat.libaio).

Upstream's SerialPortConstants.h uses uint8_t without including ; a generated_files shim supplies it and #include_next's the real header. The test opens on the libserial header first of all, so the shim is compile-time asserted, and asserts the rest over a real openpty() pair.

No CN mirror (no mcpp-res write access): plain-string upstream url per docs/cn-mirror.md.

[Sunrisepeak]

Maintainer follow-up, appended as commits on top of your branch (history unchanged):

1. b825851 merge main: the branch was behind main.
2. 6600af0 CHANGELOG: the earlier merge had dropped two existing entries (openkal-llvm-runtime 0.15.4 and openkal-linux 0.16.1 / openkal-musl 0.20.1). They are restored, and libserial now has its own entry.
3. e144221 libc++ fix + CN mirror
   • Why workspace (linux llvm) was red: SerialPort.h's public template call_with_retry uses std::result_of, which C++20 removed. libstdc++ still ships it, libc++ does not, so all three TUs failed on clang, and any consumer TU including the header would fail the same way. Upstream master still has it.
   • Why patch the header in install(): a shim cannot reach a template inside the header, and _LIBCPP_ENABLE_CXX20_REMOVED_TYPE_TRAITS would have to precede the first libc++ header of every consumer TU.
   • The fix: install() rewrites both spellings to the equivalent std::invoke_result_t<Fn, Args...>, adds <type_traits>, and asserts the match counts (2 / 1), so an upstream change fails the install instead of silently skipping the fix.
   • CN mirror: mcpp-res/libserial is created, byte-identical to the GitHub tarball. url is now { GLOBAL, CN }.

Verified locally on gcc 16.1.0, llvm 22.1.8 and llvm 23.1.3, and CI is all green. Details are appended to .agents/docs/2026-10-03-add-libserial-plan.md §10.