C++20 port 1/14: build, ownership core and allocation audit (#3065)

* Enable C++20 alongside C11 in the build

Add CXX to the project with C++20 (no extensions) and build C++ sources
with -Wextra -fno-exceptions -fno-rtti -fno-threadsafe-statics on top of
the existing warning flags. Give every toolchain a C++ compiler: the zig
Windows toolchain, the static macOS/Windows CI jobs, the static musl
image (clang++ plus libstdc++ headers) and the RPM spec.

* Add C++20 ownership and allocation core

Header-only primitives for the no-heap-after-startup design:
Result/Status, Region, ObjectPool with PoolPtr (std::unique_ptr with a
pool deleter), generation-checked Handle and asynchronous close support,
intrusive RefPtr, FixedBuffer, MonotonicArena, FixedHashMap and FixedLru,
IntrusiveList, and a bounded FixedHeap for third-party allocator hooks.

Unit tests cover each primitive (test_core_*). scripts/check_no_heap.py
rejects heap allocation in C++ sources under src/ and runs as a ctest.
clang-tidy now also analyses C++ sources and the core headers.

* Add heap allocation audit for relayed connections

alloc_counter is a preloadable malloc counter (glibc, macOS) that writes
a snapshot on SIGUSR2. measure_alloc.py warms ss-local and ss-server up,
snapshots both, relays more connections and reports allocations per
connection. It runs report-only as the alloc_audit_report ctest.
interop.peers() now yields the started processes.

* Document the C++20 port and allocation baseline

docs/cpp-core.md describes the language profile, the core primitives,
the phase plan and the measured C baseline (77-89 allocations per
connection in ss-server, 27-39 in ss-local). The compatibility contract
now keeps the C11 public header instead of a C11 code base. CLAUDE.md
drops stale libev, submodule and gnu99 references.

* Fix CI for the C++ core: libstdc++ headers, static unwinder, newer clang

- docker/static: Alpine's libstdc++ headers come with g++, and the C++
  tests' static libstdc++ needs an unwinder that compiler-rt lacks; add
  g++ and llvm-libunwind.
- FixedBuffer: drop the unused kCapacity (clang-tidy 18 reports it as a
  dynamically initialized header static).
- ss-server on Windows: manager_addr was only read in POSIX-only code,
  which newer clang rejects (-Wunused-but-set-global); say that the
  option is ignored there.

* CI: allow running the test and build workflows by hand

Stacked PRs target their parent branch, which these workflows do not
trigger on; workflow_dispatch lets each layer run the full matrix.

* CI: let clang-tidy ignore GCC-only warning flags

The compile database comes from a GCC build, whose flags (for example
-Wno-class-memaccess) clang-tidy does not know and reports as errors.

docker/static/Dockerfile

@@ -2,8 +2,8 @@ ARG ALPINE_IMAGE=alpine:3.24
 FROM ${ALPINE_IMAGE} AS build
 ARG ALPINE_MIRROR=https://dl-cdn.alpinelinux.org/alpine
 RUN sed -i "s|https://dl-cdn.alpinelinux.org/alpine|${ALPINE_MIRROR}|g" /etc/apk/repositories \
- && apk add --no-cache clang lld make musl-dev compiler-rt cmake linux-headers python3 binutils file
-ENV CC=clang
+ && apk add --no-cache clang lld make musl-dev compiler-rt llvm-libunwind-static g++ libstdc++-dev cmake linux-headers python3 binutils file
+ENV CC=clang CXX=clang++
 WORKDIR /src
 COPY . .
 ARG SS_MINIMAL=OFF
@@ -11,7 +11,7 @@ RUN --network=none cmake -S . -B /build \
       -DCMAKE_BUILD_TYPE=Release \
       -DSS_DEPENDENCY_MODE=bundled -DWITH_STATIC=ON \
       -DSS_BUILD_STATIC_LIBRARY=OFF -DSS_BUILD_SHARED_LIBRARY=OFF \
-      -DSS_MINIMAL=${SS_MINIMAL} -DCMAKE_EXE_LINKER_FLAGS="-static -fuse-ld=lld --rtlib=compiler-rt" \
+      -DSS_MINIMAL=${SS_MINIMAL} -DCMAKE_EXE_LINKER_FLAGS="-static -fuse-ld=lld --rtlib=compiler-rt --unwindlib=libunwind" \
       -DCMAKE_INSTALL_PREFIX=/out \
  && cmake --build /build --parallel 4 \
  && ctest --test-dir /build -L 'unit|vendor' --output-on-failure --no-tests=error \