fix: exit when a PHP thread segfaults as PID 1

[cyppe]

Summary

• install a Linux-only SIGSEGV fallback before Go's runtime starts when FrankenPHP is PID 1
• preserve any pre-existing non-default handler and leave non-PID-1 behavior unchanged
• exit with status 2 when Go forwards a fatal fault from a C-created PHP thread
• add a deterministic PID-namespace regression test with a timeout, so the previous runtime.raisebadsignal loop cannot hang the suite
• explicitly enable the test's passwordless-sudo fallback in GitHub Actions, where unprivileged PID namespaces are restricted, so CI cannot silently skip the regression

The underlying signal-forwarding problem is golang/go#59569. This keeps the workaround local to FrankenPHP and avoids carrying a patched Go toolchain.

Closes #2558.

Verification

• built the official Debian production runner target with Go 1.26.5 and PHP 8.4
• go test -race -count=1 .
• go test -race ./... in caddy/
• TestCThreadSegfaultAsPID1 exits 2 instead of timing out
• the same helper outside PID 1 is still terminated by SIGSEGV (shell status 139)

The focused test can be run after building the existing test helper:

cd internal/testcli
go build
cd ../..
go test -race -run '^TestCThreadSegfaultAsPID1$' -count=1 -v .

If unprivileged PID namespaces are disabled, opt into the same strict fallback used by CI:

FRANKENPHP_TEST_PID_NAMESPACE_WITH_SUDO=1 go test -race -run '^TestCThreadSegfaultAsPID1$' -count=1 -v .