linux-x64 prebuild still needs AVX2: addon C++ is compiled for x86-64-v3 #57

Problem

The linux-x64 prebuild of every published version crashes with Illegal instruction (SIGILL) on x86-64 CPUs without AVX2 (pre-Haswell Intel, older AMD, many low-end Atom/Celeron/Pentium parts). #55 removed the AVX-512 code from libghostty-vt, so 0.1.0-beta.5 runs on AVX2 CPUs again, but the README and CHANGELOG say the build now "runs on older CPUs", and that holds only for the libghostty-vt part.

Evidence (published npm tarballs, consumer install, one script that creates a terminal, feeds, snapshots, disposes)

node -e 0 alone exits 0 under -cpu qemu64 on this host, so the addon causes the crash.

VEX-encoded (v*) instructions in the addon's own C++ functions (libghostty_vt_node::* and Napi::*), counted with objdump -d: beta.0 has 0, beta.4 has 481, and beta.5 has 481. Examples in beta.5 are vpxor in TerminalWrap::BuildSnapshot and vmovdqa ... %ymm0 in DefineIndex. libghostty-vt itself is built with -Dcpu=baseline since #55.

Likely cause (inferred, not verified)

ubuntu-latest (Ubuntu 26.04) runners install Ubuntu's amd64v3 packages (see the comment in .github/workflows/ci.yml), and their g++ appears to default to -march=x86-64-v3. node-gyp compiles native/*.cc with that default. beta.0 was built before the runner change.

Possible fixes

• Pass -march=x86-64 (or -march=x86-64-v2 if that is the chosen floor) in binding.gyp cflags_cc for Linux x64, or build the linux-x64 prebuild on ubuntu-24.04.
• Then make check-baseline-cpu prove the chosen floor. Today CI uses QEMU_CPU=Haswell because Node alone dies under qemu64 on the runner, so CI cannot catch this. One option: run the check in an ubuntu:24.04 container on the runner.
• Decide and document the supported x86-64 floor (v1, v2 or v3) in the README.

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